Aperture question

ajf

ajf

ajf

Soldato
Joined
30 Oct 2006
Posts
3,067
Location
Worcestershire, UK
Been talking about aperture with a colleague and really want to check my thoughts are correct.
All things being equal would there be any image quality difference between an F4.0 lens and an identical one, except it has F2.8 max?
I said not, assuming the photo is taken at the same aperture on both.

I have always thought 'fast' lenses such as an F2.8 etc were only really advantageous in low light or other instances where a fast shutter speed was needed?
Is there a significant difference in DOF at 2.8, compared to say 4.0? This was the only other reason I thought they might be used.

If I am wrong please correct me :)

Andrew
 
Well, if you take a mid range focal length like 35mm, a full frame camera and your subject is 5m away. Then at f/4 the depth of field is a shade over 6m in total, at f/2.8 it's a shade under 4m.

The answer being, it depends on the situation, the difference is sometimes important but generally doesn't matter as if you're looking for shallow depth of field you'll shoot a faster prime instead.

I wouldn't buy a f/2.8 lens over an f/4 for the difference in depth of field myself, but then I've always argued that the primary advantage of the f/2.8 lenses is they have professional level build and image quality. Nikon don't have any seriously good f/4 options except the 16-35 VR. Canon on the other hand have the 24-105L which I rate very highly, image quality is excellent and it's a very flexible lens to own.

Back to your question - the best comparison in terms of identical lenses is the Canon 70-200 f/4 and f/2.8 versions. In this case the f/4 is as good optically and the f/2.8 advantage is limited to light gathering and depth of field. As you go to normal and wider focal lengths though, the difference between f/2.8 and f/4 isn't as useful as it is with a telephoto.
 
the DOF at a given focal length at the same aperture will be the same, The faster lens maybe slighlty sharper stopped down and have less negative attributes like CA etc however it will be marginal at best.
 
Lens design for a 2.8 max aperture requires hgiher optical quality in general so can often result in better image quality when stopped down. What you may find is a f/2.8 lens is slightly softer at 2.8 than an f/4.0 lens at f/4.0, but when the f/2.8 lens is stopped down to f/4.0 it could be much sharper than the f/4.0 lens wide open. However, a very good f/2.8 pro lens should be blinding sharp wide open or it is not really worth it.

Some of the advanatges of faster lenses is that the viewfinder is brighter and the autofocus is faster and more reliable in low light situations. On telephoto lenses you have more scope for using teleconverters (on a f/4.0 lens you can't really get away with more than a 1.4XTC, but an f2.8 lens will take a 2.0xTC, and with the faster lens with the same TC there is the potential to stop down and still have a reasonable aperture, e.g a 300mm f/2.8 with a 1.4TC could be stopped down 1 stop to f/5.6, the f/4 version will be at f/8 which is really quite slow).
 
Last edited:
if i remember rightly lenses focus while wide open, so a lens with a max of 2.8 may have better AF in lower light. so in terms of focus lock you may be a better focused image.
 
Thanks very much for all the feedback. A few interesting points I hadn't considered and some comments that backed up my general thoughts :)

I have forwarded this thread to the person I was discussing it with too.

Andrew
 
You must log in or register to reply here.
Back
Top Bottom