f/8 not the sweet spot?

[jaycobar-chay]

I took a picture last night with my Canon 10D, using my 28 prime,

mounted on a tripod, using the self timer and mirror lockup. It was

dark, so I started with f/2.8. Just for giggles because I had it on

the tripod, I ran through the aperture range, up to f/16.

 

I have always read that lens performance tapers off toward the

smaller apertures, and that the same can be said for the larger

apertures. Generally, f/8 is considered the best performer of any

lens.

 

I was amazed at the poor quality of the wider apertures and f/8 as

well. Sharpness was incredible past f/8, the one stop jump from f/8

to f/11 made a world of difference and f/16 was tack sharp all the

way through the image.

 

What's going on here? Is general camera knowledge sending out the

wrong information?

 

I've taken crops (roughly equivalent to one another) from each image

and will include the entire frame as a reference for location of the

crops. I underexposed everything, probably because the table was

black and I forgot that the meter can be fooled, but we're not

focusing on my shooting ability, we're just looking at lens

performance.

[jaycobar-chay]

Will try to post the picture, but it didn't seem to work above...

[jaycobar-chay]

This is a pisser, the form that lets you add a photo after you "confirm" your message isn't coming up.

 

Great. I wasted all that time writing the post...

[charles_stobbs3]

The depth of field would increase as you stop down. For a guess, you may have a focus problem. This would explain an object at other than the set focus distance getting sharper as you stop down.

[robert_potts1]

F16 was sharp throughout because of increased depth of field. When choosing an aperture, depth of field, shutter and mirror vibations, and shutter speed for subject movements should also be considered. The sweet spot varies a good deal and is just a general concept. F11 is often best for long lenses. F5.6 is often best for wide ones. But not always. Some expensive lenses are designed to perform well wide open. And there are several factors to consider when defining performance such as light fall off, flare, distortion, etc. There is more to this than sharpness. If you are trying to resolve lines of a target on a flat chart, f8 will outdo f16 for most wide angles. However, f16 will often be a better choice for landscapes anyway, or 2.8 for portraits depending on which devils you are fighting.

[walterh]

And then lenses can not read PN postings - so they do not know what "everybody" knows - that f8 is simply the best.

 

Jacob - now seriously - its just a rule of thumb - not a generally accepted law of physics. besides as it was pointed out another factor may have affected your image.

[neal_shields]

A lens is subject to aberations. Most get better as you stop down so the image gets sharper. However, defraction gets worse as you stop down so for every lens there is a point where the two lines cross and the improvements from stopping down are canceled out by the losses due to defraction.

 

These are all measurements taken at the point of focus. Often depth of field is more important than absolute image sharpness.

 

The F8 asumption assumes that the sensor (or film) is capiable of resolving more than the lens can deliver.

 

As film is usually capiable of resolving between 80 to 200 lp/mm it usually wasn't the limiting factor.

[dan_fromm1]

f/8 is best?

 

Tell that to my Zeiss Luminars and Reichert Neupolars, which are best wide open and have maximum apertures faster than f/8. And how am I to get the best from my process lenses, whose maximum apertures are f/9 and f/10? They don't open to f/8. Whatever am I to do, poor me that I am?

 

Just another stupid and false rule of thumb. Bilge!

[vivek iyer]

For DSLR use, you have throw the "wide open- better resolution, stop down-diffraction degradation" concept from film days out the window and far away.

 

Even when focus is accurate, stopping down always helps with digital.

 

Your observation is correct and agrees with what I find generally. Just ignore all the comments (about wide open, better performance and all that sort) here which are very valid for film.

 

If your lens will stop down to f/22-32, and you can use it (light permitting) in those aperture settings, by all means use it for even sharper images.

[jerry_friedman]

I am not surprised by your results. I shoot Medium Format where

depth of field is more apparant at much smaller lens openings. I

find that I always try to shoot at f/11- f/16 for greatest across the

frae sharpness. BUT! Hand holding a camera often requires a

faster shutter speed and my experience has been to shoot at

1/250 second as a rule. This often means that I have to open the

lens to f/8, depending film in use. F/8 should be totally sharp at

least in th center of the frame if not at all corners. So, for MF I

shoot TMAX 400 and that way hope for the best of both worlds. In

35mm, even f/8 should deliver great enough depth of field and

sharpness so perhaps you should test your lens again. Even on

a tripod there can be some shake sometimes. If you still get less

than sharp negatives, I think you may have a focusing

adjustment problem. But, keep in mind, if you are getting good

results at f/11, why worry?

[yakim_peled1]

1. It's a general rule of the thumb and not a strict rule that applies to all lenses at all times.

 

2. It is not precisely f/8 but 2-3 stops from wide open. In most lenses f/8 falls within this range.

 

3. The 28/2.8 is reputed as an excellent lens. Your test results suggest that either you had a problem with testing methodology or have a bad copy (I had a bad copy of 70-200/4 L...).

 

4. I have 3 tele primes: 85/1.8, 200/2.8, 300/4 IS. One of the main reasons I bought them is that this rule does not apply to them. They are as sharp wide open as they are stopped down. My wide primes: 24/2.8 and 35/2 only need one stop to be critically sharp and wide open is still very good.

 

HTH.

 

Happy shooting,

Yakim.