Keith,
You're not duping yourself. If the images you're getting with the 80-400 look better than the images you were getting with the 70-300, then they are better. Better, is a subjective evaluation and, since they're your photos, you get to make the evaluation.
Here's where Tony is getting the numbers he mentions:
- APS-C Coverage: The 35.9mm width of a full frame Nikon FX sensor is 1.53x the width of a 23.5mm Nikon DX (APS-C) format sensor. Another way of describing this is to say the DX format sensor is 65% as wide as an FX sensor. If we square that number, we get the comparative area of the DX sensor: .65^2 = .4225 or a skosh more than 42%. The DX format sensor covers 42% the surface area of an FX sensor.
- Image Magnification: The smaller size of the DX sensor captures images having a more narrow angle of view than would be captured by an FX sensor. That narrower angle of view has the effect of making the subject appear larger within the frame. The 1.53x crop factor tells us that the subject will appear 153% as large in one dimension (width or height) as in an image made with the same lens on an FX body. To determine the difference in total surface area covered by the subject within the frame, square the crop factor to get a result of 234%.
- If the subject appears larger so do any aberrations or defects within the image.
I am skeptical of Tony's implied message that lenses designed for use on full frame bodies suffer some special form of image degradation when used on a crop sensor body that is not inflicted upon a lens designed for use only on crop sensor bodies. The reason I'm skeptical is that the optical quality of a lens is independent of sensor size. A Sigma Art lens or a Zeiss Otus is no less excellent when it is mounted to a crop sensor body.
Pixel size plays a role in perceived resolution. A 70mm, f/2.8 lens has a clear aperture of 25mm and a theoretical resolution limit of about 4.6 arcseconds. A D610 pixel with a pitch of about 6 microns will cover an area about 17.5 arcseconds in diameter and be nearly 4x larger than the smallest detail the 70mm, f/2.8 is capable of resolving. A D7100 pixel with its pitch of about 3.9 microns will cover an area 7.7 arcseconds across and be about 2.5x larger than the smallest resolved detail.
As optical aberrations and other factors conspire to force the lens to perform somewhere below its theoretically perfect potential - no lens is perfect - the smallest resolved details in images made with either body will actually be larger than the 70mm, f/2.8's theoretical capability. But as long as that image degradation doesn't crush resolution too severely, the pixels in the D610 and D7100 sensors will still be larger than the smallest resolved detail. At least, that's my take on the issue.
Of course, I could be totally wrong, which is why I say I'm skeptical...but not certain.
In my own testing and comparison of FX vs DX camera performance, I've consistently used lenses designed for use on full frame sensors. These include three Nikkor and two Tamron lenses. On many occasions, I've gone back and forth between FX and DX bodies with those lenses, often over a span of just a few minutes. In other words, variables and conditions were essentially identical. This includes the subject, distance, lighting, focal length, aperture, etc. To be sure, five lenses is not a large sample size. That acknowledged, other than the cropping of the angle of view by the DX body, I've not noticed any image degradation that I would describe as obvious in comparison to images made just moments before using the same lenses on an FX body.
The D500, with fewer but larger pixels than the D7100, is likely to fare even better. Personally, I wouldn't be too concerned.
I'm shooting with a D7100 (until my D500 gets here). I switched from the 70-300 kit lens to a 80-400 "pro" lens. I loved the difference the 80-400 made.
So I'm listening to Tony Northrup's video, "20 things photographers get wrong," and Tony argues that since my "APS-C body only covers about 45% of the the lens, it is magnifying the defects in the lens by 200%" and I'm actually getting worse images than I would from the much cheaper kit lens.
Admittedly, I'm not photography tech savvy, but "in my mind, and to my eye," my pro lens shots look better than my kit lens shots.
Am I duping myself by buying good glass? Or should I strive for cheap glass for better images as Tony argues?
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http://www.flickr.com/photos/107641481@N02/
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Bill Ferris Photography
Flagstaff, AZ
http://www.billferris.photoshelter.com
