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Some F-Stops Are More Equal Than Others

Posted By John Sevigny On January 5, 2010 @ 12:03 am In Art of Photography | 20 Comments

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Let’s say that your 50mm lens has seven f-stops. You might think that, depth of field issues aside, your lens should be capable of making equally sharp, equally well-defined images at any of them.

And maybe that’s true — if you never print anything larger than 6×8 inches.

You see, science has its laws. Light behaves in particular ways under given circumstances, and we learn with experience that one aperture is not as good as another. Nor is it a simple question of using aperture to expand or limit depth of field.

The truth is, every lens has but a single optimum aperture that will produce maximum sharpness and definition. Not only that, but virtually every 50mm lens — Leica, Zeiss, Nikon or Canon — has at least two f-stop settings that can be counted on to produce pictures of mediocre optical quality.

All Glass Has Its Flaws

My Contax Zeiss Planar 50mm 1.7, for example, produces low-contrast, shallow depth-of-field photographs wide opened, which is to say, at f1.7. This is an expensive and high quality lens. Shouldn’t it perform well at every aperture?

No. Because light’s behavior is dictated by scientific laws, and moreover, glass — all glass — has its flaws.

A photograph shot at 1.7 relies on nearly all the glass in the lens, and any surface or engineering imperfections are going to be revealed with a wide-opened aperture. For the same reasons, I can probably forget about the idea of shooting at 2.8 if I’m looking for maximum sharpness and resolution.

Additionally, depth of field would be so shallow at f1.7 that it’s unlikely that I’d be able to get an accurately focused shot, particularly considering that this is a manual focus lens and that my eyes get worse every year.

On the other end, f16, the smallest aperture, is useless at 35mm or for DSLRs. Diffraction, a kind of distortion that happens when light passes through small holes, destroys images at f16. You might rule out using f11 for the same reason.

We are now down to three optimal f-stops on this extremely high-quality, 50mm lens — perhaps one of the best constructed ever.

With cheaper lenses, you get even fewer “good” f-stops. My advice? Don’t bother with cheap lenses. Buy the best glass you can afford and when you can, stick with the best apertures it has to offer.

So can you take good pictures at f1.7 or f16? Sure — but you can also take good photographs with a plastic Holga. We’re talking about science here, not art (though, admittedly, the art of photography is far more important than the technical crap).

From Seven to Three

So what am I to do now that I’ve narrowed down my seven f-stop lens to just three usable aperture settings?

Work with it.

Need more speed than you can get at f4? Use a faster film, a tripod to take advantage of slower shutter speeds, or a higher ISO on your digital camera.

Need more depth of field than you can get at f8? Switch to a 35mm lens, or wider if you don’t mind distortion. Photography has always been about compromises, give and take, balance in all things technical.

Don’t know the best aperture of your 35mm camera? Fall back on the old rule my father taught me back in the 1970s: the optimum aperture for sharpness and detail is about two stops away from wide opened. That is, on a lens with a maximum aperture of 2.8, you’ll probably get the best results at 5.6.

On my Zeiss 50mm, with a maximum aperture of 1.7, the “best” aperture has proven to be about f4. That doesn’t offer me a lot of depth of field — but the pictures do look punchy, bright, and tack sharp.

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20 Comments (Open | Close)

20 Comments To "Some F-Stops Are More Equal Than Others"

#1 Comment By jonlj On January 5, 2010 @ 12:39 pm

Simple but effective way to remember how to get sharper pics.
Thanks for this one.

#2 Comment By Charles Carstensen On January 5, 2010 @ 1:51 pm

Great article, thanks. I wonder how the mtf curves fit into this thinking.

#3 Comment By Rowan On January 5, 2010 @ 2:49 pm

Interesting. I never thought about it this way. I'll need to have another look at me lenses now. Thanks a lot.

#4 Comment By Shuling Yong On January 5, 2010 @ 4:02 pm

Thanks for the great advice! I'm gonna be sharing this!

#5 Comment By Ken Kollodge On January 5, 2010 @ 7:33 pm

Interesting how that works out with 35 and 6x7cm to some degree. But, with 4x5 and the Rodenstock APO lenses I have, two stops in from max might be f8 &1/2 which no one would use if you could get away with shooting at f16 outside in no breeze conditions. If you could get away with f22 and no softness from camera shake, you were in "fat city." Ansel even belonged to a group calling itself the f64 Club as I recall but then, they were likely using 8x10's.

#6 Comment By G Dan Mitchell On January 5, 2010 @ 9:02 pm

Your basic point is a worthwhile one to think about. Lenses cannot perform equally at all apertures.

But I believe you go overboard in a few places. For example, you state: "f16... is useless at 35mm or for DSLRs. Diffraction... destroys images at f16. You might rule out using f11 for the same reason."

No. On a full frame DSLR (or 35mm film camera) f/11 produces excellent and very sharp images. Yes, diffraction does decrease the maximum potential sharpness at these apertures, but the difference is very tiny - to the point of being negligible in most case. In addition, a number of lenses continue to become sharper in the corners as you stop down, even as center sharpness might decrease a bit due to diffraction. Which is sharper: the lens that gets the optimal center sharpness but noticeably softer corners or the lens that has ever so slightly less sharpness in the center but is more uniformly sharp across the frame?

That said, I agree that it is very useful to understand the real world performance of your lenses at various apertures - to know how and where they are strong/weak and to understand how this can be used to make more effective use of your lenses.

Take care,

Dan

#7 Comment By Marcel Göertz On January 6, 2010 @ 5:25 pm

It's funny to see this posted in the "Art of Photography" section.
This is all about technicalities, and not about the end result.

Dear John Sevigny, ask yourself this: why did the people at Contax bother to make the Contax Zeiss Planar 50mm as fast as f/1.7, if it performs at it's best at f/4 ? Because "best" is a flexible word, and f/4 is often not the best aperture.

The classification "best" is not just about sharpness and crispness.

Furthermore sharpness is really only one of the factors in a photo and in fact it's the most overestimated factor (except when it comes to catalog product photography stuff, but that lacks any form of photographic creativity imho)

I do hope you understand that the f-number does not only exist for getting the "best EQ" out of a lens, but more importantly for two other reasons:
- *control* the depth of field
- *control* the amount of light entering

So if you want to make a photo with a shallow depth of field in a certain composition using a 50mm lens, you'll have to go to for instance f/2 (or you move your subject farther from the background....but that changes the composition and not the DoF!)
Also, at dusk even ISO6400 is quickly too little if you keep insisting to take that shot at f/5.6 ; you'll simply need to open up that aperture.

#8 Comment By Mike Cirella On January 7, 2010 @ 12:20 pm

Good article that provides perspective about the limits of lens performance. As mentioned by Marcel, "best" is a moving target, depending upon the photographer's intent.

For large prints, as John rightly points out, it is important to pay attention to aperture selection; however, processing and printer capabilities/settings are equally important to obtain the sharpest details in the print.

Generally, the writer is spot on with the optical science, but readers should not go overboard avoiding use of stops at the extremes. Diffraction effects come into play when the physical aperture diameter approaches the wavelength of light used. But in reality the sensor pixel spacing is also important to note.

The basic formula for lens resolution (applying the Lord Rayleigh criterion) is R=2000/f#. For f/64, the resolution would be ~30 lines per mm; for f/2 it would be 1000 lines per mm. What this means is the lens set to f/64 can resolve two lines spaced ~33 microns apart. For f/2, the lens can "resolve" 1 micron spaced lines. This assumes no image degradation from other lens aberrations and is a simplistic view.

Ultra-high pixel count sensors (25 megapixels) have pixel spacing ~4.5 (Canon 5D) to 5.94 microns (Nikon D3X) (1 micron = .001mm). So you can see that somewhere below f/11 the lens resolution is roughly equal to the pixel separation and the sensor dimensions become the limiting factor. For apertures smaller than f/11, diffraction "spreads" the edge or spot, thus limiting the sharpness of detail recorded.

Lens designers try to optimize lens resolution at a particular aperture (usually f/8). As you stop down or up from there, aberrations (like chromatic aberration, coma, astigmatism, etc.) reduce max resolution. The diffraction limited aperture (DLA) is always higher (like f/16 or f/22).

This is getting complicated!

Bottom line, take a series of pictures of a detailed object with a full range of f/stops. Compare and decide for your camera sensor-lens combination where diffraction and/or your lens aberrations limit the detail or sharpness of the scene.

But don’t be afraid to use widest or smallest aperture settings. Other factors (pixel spacing, camera or subject motion, sensor noise, lens aberrations – so use expensive glass if you can afford it, defocus because the subject moved slightly, etc.) often come into play.

#9 Comment By Tammy Bryan On January 7, 2010 @ 6:38 pm

Very good information - all of it. 'Best' is indeed a moving target and different for each photog and shot.

Contax bothered to make the Contax Zeiss Planar 50mm as fast as f/1.7, so that it would out perform other lenses at it's best - f/4 for 'most' uses. Just like Nikkor made the 85mm f/1.4 so that it would rock at f/2.8.

The only exception I know of, is my Nikkor 200mm f/2.0. I often shoot that monster wide open with outstanding results. However, just like the OP, I fall back on the old rule of thumb often: the optimum aperture for sharpness and detail is about two stops away from wide open - and/or closed.

#10 Comment By Denver Engagement Photographer On January 8, 2010 @ 2:25 pm

I think it's really hard to define the "best" aperature, while it might not be Tack sharp wide open and loose contrast, the visual effect of the image might be well worth the comprimise. Getting just the eyes in focus, and having everything else blurred to a nice soft wash of color behind the subject might be just what you're looking for.

#11 Comment By Daniel Fealko On January 9, 2010 @ 10:28 am

"A photograph shot at 1.7 relies on nearly all the glass in the lens, ..."

This statement implies that less of the front element is used when stopping down and that implication is just not true. A large portion of the glass is used regardless of the aperture used. Although it is true that each image point uses only a small part of the front element at a smaller aperture, the image as a whole still relies on a large part of the front element.

It is also true that localized imperfections in the glass will affect fewer image points at smaller apertures, and therefore, less of the overall image, than larger apertures. Maybe this was what you meant, but I just wanted to expand on that point.

#12 Comment By Jeff Ball On January 20, 2010 @ 3:30 pm

Great article, thanks for posting

#13 Comment By Joakim On January 20, 2010 @ 3:32 pm

High quality words.. must be written at f/8? :D

I don't know if you know about Matt Klaskowski? (lightroomkillertips.com) He suggest in a video with dtowntv.com that you always should shoot landscape at 11 - 22... I just laughed. 11 ok, but higher, no way. Then its better to merge two images together

#14 Comment By Tim On January 20, 2010 @ 3:54 pm

Ken: LF lenses are designed to be used stopped-down further. Normally f/22 to f/32 with occasional deviation to f/45 is par for the course for lens-sharpness.

Charles: MTF graphs will probably show it; a graph is the contrast between black/white at a given resolution (line-pair/mm) at aperture. Along the X-axis you have the distance from centre of image, and then two curves in the range 0-100% for the contrast measured radially (along the radius) and saggitally (perpendicular to the radius). Normally these lines wiggle around a bit and then drop off towards 0 as the radius approaches the coverage of the lens. So you want a graph for each aperture to compare, and to be looking for the lines to be high-up % and stay consistently close together (no use being sharp radially but not the other way) as far towards the right as possible.

#15 Comment By Phil Soheili On October 23, 2010 @ 7:12 am

As a general rule I would agree to your point, but with the limits of a generalisation.. There is no way around knowing your equipment and knowing how to use it - that includes the lenses (that is your best point) and the best way would be to do a series of tests with all your lenses.

Secondly from a technical point of view resulting values (such as definition or resolution) can be compared using the numbers, but artistically seen, who can say that a particular 'flaw' of your lens couldn't make all the difference in a picture.

As is true know thyself it is also true know thee equipment.
And thanks for your article.

#16 Comment By Bjarte Edvardsen On December 12, 2010 @ 7:46 pm

I've been taught earlier that the middle aperture of pretty much every lens were supposed to be the sharpest aperture, but I guess it's hard to say such things for certain. (At least "somewhere in the middle" seems to be a rule to keep in mind.)

Thanks for your article.

#17 Comment By Tenisd On January 31, 2012 @ 9:22 am

I like to use all the lenses wide open. Would not buy a lens that is not sharp that way..
Yes, they get better when stopped down, but they need to be sharp enough on all the F stops - or they are not worth the money..

#18 Comment By Mike On March 21, 2013 @ 12:08 am

Aperture priority is the usual way I shoot.
Basing off of f8 and increasing or decreasing as needed for the particular subject matter. I do sometimes use wide open. Try not to drop below f11.
In macro shooting I have been known to use f16 because depth is the overriding issue sometimes. Moving target is a good way to put it it depends on what you are doing and how much compromise you are willing to accept.
Enjoyed the article and the comments. A good read.

#19 Comment By carl valle On May 17, 2013 @ 2:16 pm

focal length does not change dof - dof is determined only by magnification and fstop. for proof, look at the depth of field scales on any zoom lens. maximum image sharpness occurs at the focus distance at the widest aperture, increasing dof reduces sharpness at the focus point. abberations reduce contrast at wide apertures. the best overall sharpness happens where aberation are more controlled by stopping down, and diffraction is limited by opening up, this point, chosen by lens designers, and predicated by cost, is generally 2 or 3 stops down from wide open.

#20 Comment By Lanthus Clark On May 26, 2013 @ 6:35 am

Most any old lens has some kind of sweet spot where it delivers sharp photos, a better/pricier lens just has a larger range of sweetness.

I try to stick with my prefered brands own glass, but I have strayed once or twice and have always regretted it. Off brand lenses always have tradeoffs for their enticing prices.

I heartily second your suggestion to find where your lens performs optimally and work within those limitations, and keep plugging away at it until it becomes second nature.

Also remember, any camera on full auto doesn't give a hoot about where the optimal aperture is so get used to working manually or at the very least in "A" for Aperture Priority.


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