Lenses for photographing surgery

[orthopod]

Hi All

I am now the proud owner of a lensless EOS-7D. One of my major uses for this will be to take images of surgical procedures for teaching purposes(I am a specialist veterinarian). There will be no need to be able to sterilize or sanitize the lens as a non-sterile assistant will take the pictures for me.

The subject will usually be well lit by (halogen or LED) operating lights, and the field of interest will vary in size from as small as 40 mm square to possibly wanting to show an overview of the entire operating table.

So my first question relates to lens selection. Is one of the Canon zoom range appropriate for this specific application. I'd rather the photographer stayed a sensible distance from the surgical field, but they can come in as close as 30-40 cm without a problem. The EF 24-105mm f/4 L IS USM appealed to me, because i though it would probably also double as a good general purpose lens for me at home as well, but then i started to wonder should I maybe be looking at a faster lens with a bigger maximum magnification, or something totally different?

Does anyone have any experience in this area or any suggestions as to what i should look at specifically. Happy to hear any thoughts

Thanks

John

[neuroanatomist]

You may need more than one lens, but maybe not, depending on your intended use for the image (i.e. if you don't need high MP final images, you can crop quite a bit from the 18 MP image to digitally magnify your final image).

Depending on budget, the EF-S 17-55mm f/2.8 is, IMO, the best general purpose lens for an APS-C camera. It would do very well for overview shots of the OR, while the 24-105mm might not be wide enough (24mm on a 7D will frame a 7.5'x5' FoV at a distance of 8', 17mm will frame 10.5'x7' at that same distance).

The maximum magnification on the 17-55mm isn't great, though. With a 12mm extension tube, you can get to 0.45x (so a 50mm wide subject would fill the APS-C frame), but you'd need to be quite close (20cm from subject).

For the close-up shots, consider a macro lens - since you'll be shooting handheld, the 100L IS might be a good choice. You mention the subject will be well-lit, but well-lit for your eyes isn't the same as well-lit for a macro/close-up shot. There are two reasons for that. First, at close distances your depth of field is very thin, meaning you almost always need to stop down for macro shooting. Second, at close distances you actually lose additional (apparent) aperture, usually ~2-stops at 1:1 magnification, and ~1-stop at 1:3 or so, where you'd be to frame a 40mm subject. So, you may be shooting at f/11 and the apparent aperture is f/16 or narrower (assuming you bought a lens with the 7D, and take a picture of something 'well lit' at f/16 - you'll probably need a higher ISO or a slower shutter speed than you want). Also, at close distances, the IS is less effective at compensating for camera shake - the 4-stop IS system of the 100L drops into the 2-stop range at 1:1. In fact, a third reason might be that the camera+lens itself will block some of the light.

Given the above, whichever macro lens you choose, you are likely going to want to consider a macro flash. Canon makes two, the MR-14EX and the MT-24EX. The latter is better for creative shooting, as you have more control over light placement, etc. OTOH, the MR-14EX ring flash is ideally suited to the type of documentary imaging you're probably looking for - flatter light, eliminates shadows, and is smaller at the business end. If you get the ring flash, that will deliver all the light you need to stop camera shake, which is why I said 'might' above. For ~$900 total, with a goal of medical image capture, you'd be better off with the EF-S 60mm f/2.8 Macro and the MR-14EX than with the 100mm f/2.8L Macro IS and no flash.

Hope that helps...

[Busted Knuckles]

What about the 17-40 it appears to be relatively close focusing?????

[orthopod]

Hey - that helps a lot and has redirected my train of thought, so many thanks for these comments. Flash was going to be my next question - more specifically 14EX or 24EX but i guess you have already answered that question for me.

Am now thinking that the EF-S 17-55 might be the way to go, with a more appropriate macro lens for the close-up stuff.

So many choices for the novice, and so much more to think about in the age of digital SLRs (especially since the last SLR I bought came out of my allowance! at least i have a salary now, but although budget is now less of an issue, I still want to spend sensibly with an eye to developing a useful set of the best quality lenses i can afford over the next few years). I am looking forward to it

Thanks again

[Busted Knuckles]

I am going to pitch the 17-40 again. Little bit closer focusing than the 17-55 (which I own and like very much) and it is a full frame lens that will have greater practicality if you ever go Full Frame. w/ a flash the 2.8 vs. f4 issue goes away and most likely you will find the 2.8 doesn't have the depth of field to get the surgical field???? at 2.8 55 mm zoom the dof is very small close up - at 50 centimeters between sensor plane and subject the DOF at 2.8 is .79 centimeters at f4 it is 1.2 centimeters.

W/ the 40 mm focal length the DOF is 1.6 CM at f2.8 and 2.2CM at f4.

I will shut up now.

Hey - that helps a lot and has redirected my train of thought, so many thanks for these comments. Flash was going to be my next question - more specifically 14EX or 24EX but i guess you have already answered that question for me.

Am now thinking that the EF-S 17-55 might be the way to go, with a more appropriate macro lens for the close-up stuff.

So many choices for the novice, and so much more to think about in the age of digital SLRs (especially since the last SLR I bought came out of my allowance! at least i have a salary now, but although budget is now less of an issue, I still want to spend sensibly with an eye to developing a useful set of the best quality lenses i can afford over the next few years). I am looking forward to it

Thanks again

[francongphotography]

I'm a proud owner of my 17-40mm f/4..and it's an amazing lens. But since you have a 7D, and I assume you just got it..I think 17-55mm f/2.8 is a better choice. I don't know the lighting condition in an operations room (never been in one, touch wood), but having f/2.8 is still a safer bet. I bought my 17-40mm because it's cheaper and I wanted to upgrade to a fullframe. If you don't have any plan to upgrade to a fullframe camera, I'd say take the 17-55 (it'll be great even for shots outside of the operations room!), or even the 15-85mm, which had a greater magification ratio, but with a downside of variable aperture. I also agree with neuro about considering a macro lens for close up shots. The 60mm f/2.8 will be a great compliment along with a standard zoom, whichever one you end up getting. Good luck and have fun shooting with your 7D!