I'm interested as to why constant aperture lenses are so sought after? Thoughts please

UPDATE: The purpose of this thread is to thrash out ideas as to what the benefit of having a constant aperture in a zoom lens. Now obviously it's more expensive to manufacture a constant aperture lens so why the need? Surely Canon/Nikon/Olympus/Sony cannot be churning these out for no reason. Even if they did, people wouldn't buy them.

Also, keep in mind that though a constant f2.8 is always thought of the only constant aperture lenses have, this is not true. There are zooms that have constant f4 throughout its zoom, usually very long telephotos used for birding.

If indeed faster lens is better and constant aperture lenses are more expensive, surely it's better to have a 80-400mm f3.5-f5.6 compared to a 80-400mm f4 because at least you get f3.5 at the widest angle.

But instead the manufacturers released a very pricey 80-400mm f4.

After seeing the thread grow to 3 pages without much progress, I wish to share my thoughts on this.

IMHO, shooting in manual exposure mode will shed light on why constant aperture lens is always better than a variable one. And this is generally agreed to be the benefit of having a constant aperture - a constant aperture will allow you to keep exposure constant throughout the zoom range.

It is as simple as that. Yes, you can adjust shutter to compensate for brightness. You can also fool around with ISO. In fact, you need to do this every time you change focal length on a variable aperture zoom. This is time consuming and error prone resulting in the frequent zoom - adjust - zoom - adjust cycle.

If you shoot in the one of the PAS/scene modes, the camera adjusts the settings for you but you open yourself up to the mercy of the camera which can result in camera shake (too low a shutter) and exposure errors (metering issues). But again the problem is inconsistency.

Thanks to all the seniors which participated in the discussion. I hope the newer folks entering the realm of DSLRs will come across this thread in the future and that it will help them answer this nagging question

Also the f-number is derived from the (focal length) / (size of the diameter of the lens opening).

"Constant Aperture Lens" is so long-winded... they are normally referred to as Prime Lens.

A long time ago, prime lenses offered the sharpest optics for the lightest weight for a given focal length vs what a zoom lens can offer. They simply have lesser optical elements to distort and degrade the light that passes through them. Today, that is still mostly true, but zoom lens design has become so advanced that mid to high end zoom lenses can match the resolving power of a prime lens for any given sensor resolution in consumer cameras.

But usage of prime lenses trade off flexibility for image quality. You have to zoom in and out with your feet, and if there's no room to move backwards or no time to move closer, you end up with a less-than-optimal crop of a shot.

errr i am noob in photography, but i think i share my idea with u.

talk about pc overclocking. if pentium-D needs extra 0.5v with EXTRA POWER COOLER to o/c a lot compare to another pentium-D which can o/c a lot at stock voltage, which 1 u prefer? i bet u prefer the 2nd one, right?

same thing as the lense. a lense which can maintain same aperture throughout the whole focal length will be very much welcomed compared to those that aperture reduced when focal length increased. of course price will be different.

just my half cent opinion. need pro to confirm.

Are you sure? Can anyone else comment about this? Constant aperture lens = prime lens?

There are also zoom lenses with constant apertures such as the 12-24mm @ f/4, 14-24mm @ f/2.8, 17-40mm @ f4, 17-55mm @ f/2.8, 24-70mm @ f/2.8, 28-70mm @ f/2.8, 80-200mm @ f/2.8, 70-200mm @ f4, 70-200mm @ f/2.8, etc..etc..

They can't be considered prime lenses though.

Dude, I guess you misunderstood, Prime lens is not the same as Constant Aperture Lens.

Constant Aperture Lens could be either Prime or Zoom lens, meaning that the aperture is fixed.
Example of Constant Aperture Zoom lens Canon 70-200mm f2.8 L
Example of Constant Aperture Zoom lens Canon 200mm f1.8 L

http://www.google.com/search?hl=en&q=constant+aperture



Incorrect...
You need this : http://photonotes.org/cgi-bin/entry.pl?id=...perturezoomlens or http://www.cs.mtu.edu/~shene/DigiCam/User-...erture-990.html

@cjtune, I think the others ideas are more similar to mine. I too think perhaps you are confused between fixed focus lens and fixed (constant) aperture lens.

I tried googling (thanks for the links) but I still can't find a strong reason as to why constant aperture is better than a variable aperture.

Ideas? I'm a n00b. Humor me.

Oh my bad.. yeah, there are constant aperture lenses which are not prime lenses.

Simple, take example a kitlens with 18-55mm f3.5-5.6(Non-fixed-aperture-zoom-lens) compared to a Tamron 17-50mm f2.8(Fixed-aperture-zoom-lens)

When you zoom your kitlens to full zoom e.g. 55mm, you'll notice your aperture will go lower and lower(which is bad) and it'll be f5.6 @ 50mm.

On the other hand, if you zoom the Tamron, the aperture will maintain at f2.8 even though it's @50mm. Meaning you still can use f2.8 @ 50mm but the highest aperture you can go for kitlens is f5.6

That's OK. We'll learn together.

Come on sifus? I need help here.

For thread starter only, a similar question was being asked b4:


I'll extract what i've just said at there:

Say I have a constant aperture lens... I shoot at F2.8 (Tamron 17-50mm F2.8), y would i need to compensate the aperture when I zoom in and out? Aperture may remain constant at F2.8, while changing the Shutter only (practically, kind of like shutter priority)... However, when I want a certain aperture say... by going F2.8 @ say 50mm (Tamron 17-50mm F2.8) I could create more bokeh at 50mm... however, if your lens, say any kit lens, at 50mm (what is the max aperture?? Maybe F4?) Not to mention the slower shutter speed that you will need to use (to get back the same exposure that of @F2.8)... cuz @ F2.8 more lights = faster shutter... while @ F4.0 the shutter speed is definitely slower than the shutter speed @F2.8.... thus, if you're not capturing fast moving objects, you'll be pretty safe... but if you're shooting 'actions' ... that slowness of the shutter speed makes an insurmountable difference....

In other sense, if you have a constant aperture lens, say a F2.8 or F4... it literally gives u more versatility on the "MAXIMUM" aperture that you want to use... (and bokeh)

Another thing to consider is the
70-200 F2.8 + x1.4 TC = 280mm (max) @ F4.0... which gives u more light compare to normal 70-300 (tele-end @ F5.6)... cuz u can have F4 through out the whole range.... while not the 70-300...

Q : What's a benefit of a constant aperture lens?
A : er...... constant aperture la.

or do you mean what's the advantage of having constant aperture?

OK. So a constant aperture lens can remain bright at longer telephotos. So this will allow high shutter speed to avoid camera shake then? I think I can understand that.

Now what if I got a D3 and I can do ISO 6400 without noise. Can't I just set it to AUTOISO then just zoom away? My shutter speed would remain relatively high. So I get benefit of a bright lens (f2.8) @50mm too?

What about lenses that are constant aperture but are slow e.g. 80-200mm f4.5 (or something like that). Do these offer benefit over something say 18-70 f3.5-f4.5?

Actually, with the age of electronically controlled lenses, does it matter to have constant aperture mechanically? The body can set the f-stop to whatever number that is higher than the largest aperture in the entire focal length zoom. Probably an artefact of a bygone era? But would be useful for Auto or fully manual, non-TTL flashes.

Thanks vX-2 for the good links. Hope that will help you guys in knowing the advantages in having fixed-aperture-lens.

Yes, I guess that's a better question

What's the benefit of a constant aperture???

Read this
Good explanations.

That's only because the lens has much bigger lens elements to give you a 'brighter', 'faster' f-stop number at the longest telephoto focal range.... which would mean a lower f-stop (and hence larger relative aperture size?) is actually available at the lower focal range had it not been limited to be constant?

Read my post and viking's one... i edited mine a while ago.



Nah... you would lose the bokeh of F2.8...


"80-200mm f4.5" .... but it's a different range that you're comparing though?

but say, it's a 18-70 F5.6 vs. 18-70 F3.5 - 4.5... then here the latter one would be better... cuz you certainly want more light, which would give u faster shutter speed, so that when u're capturing object that is moving, u'll be able to "freeze" it.... of course, there's also another technique call panning... but even for good panning, u'll still have a "freezing" point in the pict...

Wah, too much information to understand in that thread la. Can someone summarize?

If I have a constant 80-200mm f4.5 lens and a 80-200mm f3.5-f4.5, which one would you prefer (or does it make a difference to you)? Note both lens might not exist, I just wanna keep the focal length similar and differentiate between the apertures.

Do any of you shoot exclusively in manual exposure?

Yes.



No point increasing your ISO, if your lens doesn't has fixed-aperture, it'll automatically increase as you zoom.



80-200mm f4.5 will remain at f4.5 all the way.

18-70mm f3.5-5.6 will go to f5.6 @ 70mm

So if the camera automatically increase ISO (let's assume no noise here), then I don't need constant aperture? Since the ISO will compensate and provide enough shutter speed to keep camera shake at bay?



I think you mistakenly read my lens comparison. What if max aperture is f4.5 for both lenses?

Or lets make it simple. Between 80-200mm f4.5 lens and a 80-200mm f3.5-f4.5, which one is better?

what's so difficult about vx-2's post la? it's very clear already.

with constant aperture, you can have better bokeh.

not only that, when you zoom, the constant aperture allows your lens to absorb more light, thus able to use faster shutter speed - it could mean the difference between a successful / failed shot.

then you have the Depth of Field.

try using not fixed aperture lens, after you TELE - it can be frustrating to see your aperture go up in number (mean it turn smaller) thus you have to compensate this loss of light by either increasing the ISO or slow the shutter or both.

Mm, could you give me 2 lenses which actually exists and I'll tell you which has more advantages.

max or min?

the lower the f number, the wider the aperture is.

so i think you should be asking about the maximum aperture opening rather than max aperture at f4.5 cos we won't know whether you mean the maximum number (as highest in number) or maximum as in opening.

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Added on April 15, 2008, 12:42 am

aiyah no need he give you

Canon EF-S 18-55 f3.5 - 5.6
vs
Tamron 17-50 f2.8

*edited sorry la. let other people explain. cos i a bit busy now but only reply here and there. i think those 2 actual lenses are good for comparison.

goldfries, let's leave DOF out for now. Assume DOF is not important (it can be present or not, don't matter).

Oops, sorry about the ambiguity about 'max aperture'. I meant the widest aperture (aka brightest).



Hard to come up with exact lenses that match. There's a 80-200mm f4 and there's a 18-70 f3.5-f4.5...

Do you guys shoot in manual exposure?

ebernie

Wider / Higher aperture / Smaller f number = f1.8
Smaller / Lower aperture / Bigger f number = f5.6

ok. now how is DOF to be left out when you're asking about advantage of fixed-aperture lens? DOF IS relevant as long as aperture is involved.

OK, maybe I put it wrongly. Let me rephrase to, "OK, benefits of DOF noted".

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Added on April 15, 2008, 12:46 am

Anyone knows how is the f number derived? Can give an example calculation?

hehe ok.

btw the f-number, how to tell you la how it's derived. no one actually goes an measure the opening and tell you "oh, this is f2.8 lens".

Hmmm... but surely the manufacturer has to somehow determine the f number right? Of course I won't go and measure it

Just wanna know why certain lenses f2.8 some f1.4 some f5.6... you get me right?

Cause they are built to be like that?

this is the type of things that people shouldn't ask in so detail. like how the mm on the lens indicates focal distance.

you find out already can do what? i rather be asking how to use studio strobes properly to highlight the features on the face while adding some colored light effects on part of the skin, clothing and hair without spoiling the image captured.

I see. You are right. I shouldn't have diverged from the original question. Let's go back to why constant apertures are better. Let me go further - why is a constant aperture is essential in getting the perfect exposure?

There's one more question unanswered - do you guys shoot in manual exposure exclusively (or at least 80% of the time)?

ebernie,

For non-fixed-aperture-lens, take for instance a Siggy 70-300mm f4-5.6

@ 70mm - The widest aperture you can use is f4
@ 80mm - The widest aperture you can use is f4
@ 10xmm - The widest aperture you can use is f4.5
@ 15xmm- The widest aperture you can use is f4.5
@ 2xxmm - - The widest aperture you can use is f5.6
@ 300mm - - The widest aperture you can use is f5.6
Sorry, I don't know the exact one, I'm just giving you an example how it increases.

Why is it bad for the f number to increase?
- You don't get the bokeh you want. f2.8's bokeh is better. (though some people can easily be satisfied with f4)
- You might not have sufficient light, low shutter speed, and picture might get blurred.

because when there's limited lighting, these wider apertures give you the advantage of getting MORE light through your lens.

in doing so, it gives more headroom to ISO and shutter speed. meaning you can use better ISO (to reduce noise) and faster shutter (to reduce shake) thus resulting in better picture in the end.

I dunno wat wat fixed aperture n all. I oni know my 17-50 f2.8 is very the roxors soxors n very guten pictarz sharpens.

I see. Correct on all counts. Smaller aperture means you lose the DOF and also the amount of light. But again, assume for bokeh matters not here (meaning shallow DOF or otherwise, it won't affect our pic), and I can increase ISO, does that make a fixed aperture lens irrelevant?

So what if I built in enough glass to get me f/4 @300mm?
If that was done, why the heck should I limit myself to f/4 @70mm? Why not f/2.8 since the optical real-estate is already there.

That's the benefit of bright lens, not constant fixed aperture lens. Some lenses are fixed @ f4.

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Added on April 15, 2008, 1:02 am

Good question.

ok. you clearly don't understand it.

compare a 70-200 - f2.8 all the way
vs
a 70-200 f2.8 - f5.6 (yes, this is a make-believe lens)

at 200mm range, the fixed focal length lens still can use f2.8 - the amount of light that comes in maintains. the one with varying aperture on the other hand has aperture reduced to f5.6, thus resulting in less amount of light, so a change in ISO and / or shutter speed is required.

Hmm... but did the sharpness is due to the fixed aperture? I think not because the 18-50 Sigma f2.8 is said to be less sharp than the 17-50 Tamron f2.8. Both are constant apertures, but one is less sharp than the other.

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Added on April 15, 2008, 1:07 am

goldfries, again let's not compare lenses that are brighter than the other. Say there's a zoom that is 70-200 f4 the another 70-200 f3.5-f4. Which one is better? Why?

Well, it's just like asking, why a BMW can go faster compared to Kancil when both are cars. Sometimes it depends on the built of the lens as well.

Build of the lens affects sharpness? So sharpness is not related to constant aperture then?

Oh, still got one question unanswered - no one shoots manual here?

Both applied. Certain lens like 50mm f1.8 is not that sharp compared to 50mm f1.4 due to the build quality. Also, a 70-200mm f4 @ 70mm is sharper than a 70-300mm f4-5.6 @70mm eventhough both aperture are at f4 @70mm.

did you know that many F4 lens are sharper than F2.8 lens? clearly it's not a matter of aperture. it's the build quality.

furthermore, majority of lens aren't sharpest at their widest aperture.



now that you put the aperture so close in your question - i'll say up to you la. you want wider aperture at @70mm or you want wider aperture at @200mm

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Added on April 15, 2008, 1:12 am

i shoot in manual. and many of us do too. what is it about manual that you want to ask ah?

Also, I think before this discussion proceeds, need to get into the mindset that constant aperture does not mean constant BRIGHT/WIDE aperture.

Because I'm not asking about benefits of having f2.8 all throughout the zoom. It's brighter. I know its benefits.

I'm also interested to know if there is a benefit of having say a f4/f5.6 throughout the zoom range.



Because shooting in manual shows the benefit of having a constant aperture

I only brought up the relationship between sharpness and constant aperture because ianho brought it up. I think it's not entirely true as well. Regarding f4 lenses being sharper than f2.8 lenses, it really depends on whether that lens is widest @f4.

Anyone knows why usually @widest aperture, the lens usually produces softer images?

If you do read my post, you'll know that I shoot in Manual mode most of the time...

Manual Mode? I do, especially when I wanna snap my own pictar using tripod.