1:1 Macro w/ digital SRL?

[mjs1973]

I'm thinking about purchasing the Canon EF 100mm f2.8 USM Macro lens as a late Christmas gift for myself. I have read the reviews on this lens and everything sounds great. My question is, what happens with the 1:1 ration that this lens can produce when put on a digital SLR, in this case the DRebel. I'm assuming that with the 1.6x sensor issue (for lack of a better term), the 1:1 ratio will not be achievable with my digital SLR. How small of a ratio can I expect to get with this lens and a digital body?

[Lionheart]

I'm thinking about purchasing the Canon EF 100mm f2.8 USM Macro lens as a late Christmas gift for myself. I have read the reviews on this lens and everything sounds great. My question is, what happens with the 1:1 ration that this lens can produce when put on a digital SLR, in this case the DRebel. I'm assuming that with the 1.6x sensor issue (for lack of a better term), the 1:1 ratio will not be achievable with my digital SLR. How small of a ratio can I expect to get with this lens and a digital body?

We use this lens a lot in the practice. Of course you'll get 1.6:1 (or 1to 0.625) ratio at the closest focusing distance, which is a plus,for magnification, unless you're actually trying to achieve an exact 1:1 image on sensor ratio. This was initially a problem for my practice since AACD accreditation photos were a required 1:2, 1:1, and 1:10 ratios. However, if you look at the distance/ration scale on the lens barrel, there's not much as far as marking the various ratios to begin with, so we just did some rough math, compensated for the crop factor, then eventually decided that was ridiculous since the distance and ratio guage aren't exacting in the first place, and just went with our calibrated eyeballs (we imagine a 35mmx24mm frame in front of the teeth we're about to photograph ).

[Lemming51]

The lens's magnification ratios apply to the image the lens projects on the film/sensor plane. It is the same whether the lens is used on a 35mm film camera, 1.3x, or 1.6x digital SLR. The 1.6x digital sensor is simply 22.5mm x 15mm cropped from the center of the 43.3mm image circle (to cover 36mm x 24mm film frame) the lens provides. When focused to 1:2 or 1:1, that cropped image is still half- or life-size, not 1.6x larger or smaller.

[Lionheart]

The lens's magnification ratios apply to the image the lens projects on the film/sensor plane. It is the same whether the lens is used on a 35mm film camera, 1.3x, or 1.6x digital SLR. The 1.6x digital sensor is simply 22.5mm x 15mm cropped from the center of the 43.3mm image circle (to cover 36mm x 24mm film frame) the lens provides. When focused to 1:2 or 1:1, that cropped image is still half- or life-size, not 1.6x larger or smaller.

You are both correct and incorrect. My diatribe is based on capturing the same perspective on sensor as you would on 35mm film with regards to meeting the AACD requirements of photographic documentation for accreditation. Yes, the cropping factor does not affect the actual ratio of image to life size when viewed on a print that is the exact size of the sensor in question. The magnification factor alters what is framed in the final image. When shooting 1:1, or 1:2, or 1:10 per the AACD guidelines, the framing of the teeth in question changes radically when the ratios are set on the lens when going from 35mm film to digital sensor that is smaller than 35mm. In order for the proper framing of the teeth to be viewed to be standardized for evaluation by the examiners, all submissions need to be standardized using set ratios. Unfortunately, that standard is based on a 35mm film plane. Setting the macro lenses to the same ratio does not capture the "standardized" requisite image for evaluation by the examiners and the cropping factor will leave out the required landmarks for judging. Therefore, either you do some math and use your calibrated eyeball to determine where to set your macro lens for a 1:1 or 1:2 "standardized" image, or you can use your calibrated eyeball to "frame" what should be seen in the image by imagining a 35mm frame over the patients teeth.
btw, the AACD=American Academy of Cosmetic Dentistry. We have four very serious dentists in this practice, all serious about the level of dentistry we practice, and all eager to be the first in the group to be accredited (not an easy undertaking I assure you) with the AACD. Any dentist can be a member, but few actually pass the accreditation process the first time.