f-stop comparison

[John Chardine]

This may be more appropriate in another forum. Please move if necessary.

I've been noticing soft images with my Canon 100mm f2.8 Macro (non-L) at f32 and offer this comparison. Focus point is on flower part closest to camera and was not touched between exposures. No processing done here except cropping. Notice how soft the object is at F32 compared to f8. Other techs below.

Is this diffraction in action or maybe focus shift? If diffraction, which I suspect, how do you obtain sharp images at f32?

Model: Canon EOS-1D Mark IV
Lens: EF100mm f/2.8 Macro USM, 100 mm
Program: Manual
ISO 800, 1/30s f8 and f32
Flash: on
tripod mounted, mirror up, self timer on 2s

[Desmond Chan]

Hmmm...I think you've got a problem, John. I'll let Mike tell you about f32, which he uses a lot:

http://www.mikemoatsblog.com/

Search under "Categories" (on the right about half way down) for "Diffraction".

Your f32 shot looks out of focus to me.

[John Chardine]

Which suggests focus shift but that problem is usually a feature of fast lenses in the f1.4 range. I'll do a few more tests today.

[Ken Childs]

The DLA for each camera is different.....yours starts at f/9.1 so it will be hard to get something tack sharp at f/32 but as Desmond said, "you've got a problem". This isn't just the normal problem you get with a small sensor and lots of megapixels.

Try hooking the camera up to a computer and test shoot using Canon's EOS utility. Everything will be nice and big so you can see what happens when you change aperture settings.

[John Chardine]

Thanks Desmond and Ken. I'm an optimist so I'm hoping I don't have a problem! Here's a test at ISO 100 with the 1DIV at all whole apertures from F2.8 to F32. All the rest is constant. Techs below. These are all at 100%, no sharpening, and are of the extremely small writing on a Canadian bank note (maybe there's a better target for this sort of test).

Image quality jumps from f2.8 to f4, which is expected, then improves a little each stop through to f8, then degrades to f32.

it would be very interesting to see a similar test with another copy of this lens and the new L-series version.


Model: Canon EOS-1D Mark IV
Lens: EF100mm f/2.8 Macro USM
Lens almost at closest focusing distance
Program: Manual
ISO 100, 1/1000s, f/2.8-f32
Exp. comp.: 0.0
Flash: on
Tripod mounted, flash as main light, mirror up, self timer, cable release

[Steve Maxson]

Hi John. This has been exactly my experience with both the Canon 100 and 180 mm macro lenses at f/32 - which is why I don't shoot at that aperture. In your first set of images, I don't see anything that is really sharp in the f/32 image - if there was a focus shift (unless it was front focusing), you might expect some other part of the image to be relatively sharp. Perhaps a more interesting question is how sharp the f/32 images are after sharpening. Does applying more aggressive sharpening result in an acceptably sharp final image? Clearly, some folks are achieving quality results at this aperture.

[James Fuller]

Very Interesting thread, thanks for sharing, I am going to be more careful about going past f16 now!

[John Chardine]

Thanks Steve. I feel a little better about this!

Do you see this kind of fall-off with that fantastic MP-E 65mm f/2.8 1-5x Macro of yours?

[Steve Maxson]

Thanks Steve. I feel a little better about this!

Do you see this kind of fall-off with that fantastic MP-E 65mm f/2.8 1-5x Macro of yours?

The MP-E lens only stops down to f/16 so I'm not seeing the same degree of sharpness fall-off. (Maybe there's a good reason why it only stops down to f/16.) :S3:

[Mitch Haimov]

This has been exactly my experience with both the Canon 100 and 180 mm macro lenses at f/32 - which is why I don't shoot at that aperture....Perhaps a more interesting question is how sharp the f/32 images are after sharpening. Does applying more aggressive sharpening result in an acceptably sharp final image? Clearly, some folks are achieving quality results at this aperture.

I've been using Canon's 180 mm macro for years and have had no problems at f/32. By coincidence, the image in the thread I started yesterday was made with that lens at f/32 with a full-frame sensor. Excessive sharpening was not required, and the original capture was significantly sharper than your f/32 examples here. I don't have anything else technical to add (DLA? 1DIV? :2). But, if you do determine that there is a problem with your current lens and decide to replace it, I do recommend something in the 180-200mm range. They offer three benefits IMO: cleaner backgrounds due to smaller angle of view, greater working distance, and a tripod collar.

I'm curious to see what others' experiences have been with small apertures on macro lenses.

[Desmond Chan]

Personally, I expect even with diffraction, something should look in-focus and that sharpening in pp should negate the effect of diffraction. For comparison, I did a quick test shooting hand-held using built-in-camera flash as light source from the target 11.8 inches away.

Nikkor 105f2.8 VR on D700. ISO 400 f36 1/200s. AF used as shot hand-held. Original in RAW converted to JPEG for posting purpose.


This is the whole photo (no sharpening) :

http://farm6.static.flickr.com/5264/5628329876_065c7d63ed_b.jpg


This is 100% crop in Capture NX without sharpening:

http://farm6.static.flickr.com/5264/5628329736_213f7d20fc_b.jpg

[John Chardine]

Looks good Desmond. You are using a FF sensor which would reduce the effect of diffraction somewhat. No wild flowers out there in BC???! Actually your subject is good with lots of detail.

To answer Steve's question, I can make reasonable images at F32 if I process normally but the 100% crops with no sharpening tell the tale. This is cropped about 50% of original size. Each anther (yellow blob) is about 1mm length-wise.

Model: Canon EOS-1D Mark IV
Lens: EF100mm f/2.8 Macro USM
Program: Manual
ISO 400, 1/30s, f/32
Exp. comp.: 0.0
Flash: on

[Desmond Chan]

You are using a FF sensor which would reduce the effect of diffraction somewhat.

All right then. Regardless of what aperture I use, I find the file from D700 looks sharper than my D300 anyhow.

OK, a couple of shots from my D300 for comparision. Manual exposure mode. ISO 800 1/160s. Again, R1C1 flash system as primary light source. One light in manual mode and the other in TTL. Camera on table-top tripod. AF. Originals in RAW converted to jpeg for posting purpose. 100% crop from Nikon Capture NX 2 (I have a feeling Camera Raw applies some sharpening so decided not to use it).

Looks like mine looks better than yours, John :S3: :cheers:

Kidding aside, here they are. f57:

http://farm6.static.flickr.com/5062/5628127591_d75cd7b82c_b.jpg


And f32:

http://farm6.static.flickr.com/5029/5628128147_faa866f207_b.jpg

[John Chardine]

Thanks Desmond. Impressive f-stop: f57!!!

[Roger Clark]

Hi John,

Sorry I couldn't respond earlier--I was out photographing (birds).

A couple of things to add to the discussion.

At f/32, the diffraction spot diameter is about 41 microns. With 1DIV pixels only 5.7 microns, that means the diffraction spot diameter is almost 8 pixels. On a D7 or D700 (8.46 micron pixels), it would be less than 5 pixels. So the D3, D700 images would look sharper at the pixel level, but actually have less detail than a 1DIV image (given the same lens).

But your image is indicating another issue. It does appear to have a focus shift as you stop down. This is an indication of spherical aberration, and that might be the reason your image is not as sharp. Did you have an extension tube in place? If so, that would increase the magnification and make the lens not work in its designed zone, which could contribute to the spherical aberration.

Roger

[John Chardine]

Thanks Roger. So you think the fall-off I'm getting at f32 is more than I should expect just from diffraction? I may try some contraption to check focus point to see if it does shift at f32, not that I can do much about it (change lenses, shoot at f8 and stack images with different focal points).

[Roger Clark]

Thanks Roger. So you think the fall-off I'm getting at f32 is more than I should expect just from diffraction? I may try some contraption to check focus point to see if it does shift at f32, not that I can do much about it (change lenses, shoot at f8 and stack images with different focal points).


John,
(I flew home today.)
Another way to check for spherical aberration is to make time exposures of distant city lights or stars. Bloated bright stars/lights are an indication of spherical aberration. Of course that doesn't work for close-up images. If you have a ball bearing (or a few of them, then you could reflect a light and do some exposures to see the response. Ball bearings create a very small spot, sort of like catchlight in an eye.

But a focus on a ruler set up diagonally and images at various f/stops would show the focus shift due to spherical aberration.

Roger

[Roman Kurywczak]

Hey John,
late to the party.......and as a guy who regularily shoots at f/22-32.......there could be other issues! I have the Sigma 180mm macro....not the Canon......but how and where are you focusing? Try this same test using "live view"......magnified 10x. Critical focus is not always the easiest to asses through the viewfinder!

[John Chardine]

Thanks Roger and Roman. I'll try the point of light test. Roman- I totally agree and always use Live View with magnification.

I'm going to have access to the 100mm f2.8 L soon and I'll test that and get back.

[Anita Bower]

.....but how and where are you focusing? Try this same test using "live view"......magnified 10x. Critical focus is not always the easiest to asses through the viewfinder!

I'm late to this discussion also, and don't understand all the technical talk. However, my first thought upon seeing the OP was that the focus of the f32 image was not on the part closest to the lens.

[John Chardine]

Hi Anita- Sorry I missed your last post. In the OP I was very careful not to change the focus point between the two images.