issues with lower aperture gone in 7d sensor?

[Marty Hudzik]

Okay. I am going to reveal my ignorance on this subject now. I have been shooting with 1/3 sensors for around 15 years in the video realm. So all I have know is a world of shooting in the f1.8-f5.6 range. If I let the lens stop down further the image would lose clarity as there was some issues with diffraction of the light and the images would be severely diffused and soft.

Enter the 7D. I see a lot of still photography done at what I would consider extreme Fstops as low as f22. I needed to shoot in broad daylight the other day and found myself stopping down to f11 (no ND filter on the lens at this point) and my spider-sense was tingling. With my XL-h1 the sharpness and clarity of the image would surely be compromised in s major way. I just kept thinking this was wrong and that I needed to open up more to get a sharper image.

The shot in question actually turned out "okay" but the exposure was a little off since I was using the LCD in broad daylight and could not accurately gauge the lighting.

Is this something I need to concern myself with using this camera? I assume that if photographers shoot at these extreme settings and get sharp images I should be able to do this with video too....right? Of course I will be using my ND filter for scenarios that require a shallower DOF but sometimes I want that deep DOF of video. (never thought I'd hear that phrase come out of my mouth!)

Any advice on this optical anomaly as it relates to the 7D would be appreciated.

[Ted Ramasola]

Marty,

This issue you describe sounds like a lens specific problem. Remember that the 7D is interchangeable lens.

Your images will only be as good as your glass. Try to see what lens you want to put on and look for reviews over at dpreview.com, or other SLR related review sites, regarding the image quality of that lens.

Each lens for DSLR have different character.

[Marty Hudzik]

I used 3 different lenses on my XLh1 and it was there on all of them as well as users with HVX and JVC. I am not saying the lens is not a factor but this has been discussed with most HD prosumer class cameras. I am inlcined to think it is related to the sensor size more than the glass. I'm sure Chris H. knows the answer to this and will weigh in at some point.

Thanks

[Mike Calla]

Your right about the 1/3 camera and stopping down... My first experience was XL1's "must use" settings to get the clearest picture. Then as i started to buy Sony HDV cams, those too had little quirks about what focal length, iris, gain etc settings to use to acheive the best results!

Yesterday, on a sunny (but always hazy and humid in Shanghai) day exterior, on my 7D i stopped down to F11.

As long as you have decent glass, no worries!

[Daniel Browning]

Originally Posted by Marty Hudzik

Enter the 7D. I see a lot of still photography done at what I would consider extreme Fstops as low as f22.

Indeed. if you take a still with your 7D at f/22, and carefully downsize it to 1080p, you'll find it is just as sharp as the XH-L1 at f/4.4.

As far as diffraction is concerned, f-number scales in perfect proportion with sensor size. In other words, f/22 on a large APS-C sensor has the same amount of diffraction as f/4.4 on a 1/3" sensor, when all else is equal (angle of view, viewer CoC, etc.). There are other ways to look at it too. One is: diffraction scales with DOF for any given composition. Another is that diffraction scales with effective iris (or "aperture") diameter.

But there is a difference in sharpness because they are sampled, filtered, and post-processed differently: the 7D has a weak OLPF, line skipping, Bayer CFA, and DIGIC; while the XL-H1 has no filter (AFAIK), Frame mode, 3-chip, and different processing.

Originally Posted by Marty Hudzik

[...] found myself stopping down to f11 [...] I just kept thinking this was wrong and that I needed to open up more to get a sharper image.

The 7D video mode does start to lose sharpness due to diffraction at f/8, but even at f/11 it's still pretty sharp. The reason is line skipping. If the 7D read the entire sensor instead of skipping two out of three lines, then f/11 on the 7D would be just as sharp as f/2.2 on a 1/3" Bayer camera.

Originally Posted by Marty Hudzik

Is this something I need to concern myself with using this camera?

One thing to consider is sensor dust. The slower the f-number, the more dust that will show up.

As for diffraction on the 7D, personally, I welcome it. When the 7D image is sharp, it has very high microcontrast (AKA "aliasing"). Many viewers love this microcontrast, but to me it is a very ugly artifact: it's a matter of taste. Diffraction helps to reduce this. But if you want the optimal sharpness, then stick to f-numbers below f/8.

Originally Posted by Marty Hudzik

I assume that if photographers shoot at these extreme settings and get sharp images I should be able to do this with video too....right?

Yes -- after you subtract about 1.3 stops to account for line skipping.