Macro close focus questions

I need some guru help. Assume a macro lens (new 60mm Canon for instance) has a close focus of 7.9" and this is where 1:1 occurs in air. What happens to that distance housed and underwater. Do you get 25% more distance to close focus, e.g. 12 inches 1:1? Assume flat port.

Remember from your OW training - it looks closer but the distance is the same. The focal length is magnified by the RI water, so the FL is now 60 times 1.33 (almost 80 mm) and the maximum reproduction ratio is now 1.33:1.

Cheers!

Tom

Yes, in a flat port thats true, not in a dome port. A dome corrects for appearent magnification keeping your lens the same U/W as topside.

Re-read the original post.

So say your 60mm lens has a close focus of 8 inches (for example). Tom, are you saying that when you shoot it behind a flat port with a subject at close focus, the lens will "think" it is focusing on something say 8*1.33 =~ 10 inches away?

Interesting.

Cheers

James

Actually, it is a first order approximation.

The full formula from Mertens (1970) p. 148 is as follows:

Apparent object distance = 1/RIw x actual distance in water + 1/RIport x (port thickness) + lens-to-port distance

RIw = 1.33; RIport ~ 1.5 (glass)

Remember to always be consistent with measuring units to avoid the NASA 'problem'

I am not sure what you mean by a lens 'thinking'; maybe apparent distance, this is what one perceives behind a diving mask. Things appear closer than what they actually are. A problem for aiming a strobe as you know.

T

Tom, FYI....I was trying to access your website, but the link doesn't work

Karl

I would assume that the Canon 60mm macro's 1:1 macro capability already factors in the 1.6x magnification factor since this lens was designed for and can ONLY be used on EF-S mount Canon cameras with a 1.6 factor. This lens is in fact a 96mm macro with a 1:1 repro capability. In the same way, the Canon EF-S 10-22mm is actually a 16-35mm lens. These lenses can never actually be used at 60mm or 10-22mm (because they aren't designed for mounting on 35mm or FF-sensor cameras). It's a different story with say, the Canon 100mm macro. It achieves its 100mm focal length and 1:1 repro capability on a full-frame camera, and thus effectively should achieve even higher apparent focal length and magnifciation when used on a Canon camera with an APS (1.6x) sensor. Right?

Sensor size (that is, crop factor) does not affect magnification.

Magnification depends only on true (not "effective") focal length and distance to subject.

Full frame and DX sensors --both with 60 mm lenses x inches from the same fish -- have the same magnification. The DX sensor just has a smaller field of view because less of the image circle falls on the sensor.

In my opinion, effective focal lengths multipliers cause a lot of confusion... I think it'd really be more meaningful to speak of field of view divisors.

My 2 cents.

All the best,

Chris

Tom,

When a digital camera takes a photo, it records EXIF with information about the shot - sometimes including zoom, distance to focus point, etc.

When I say the camera "thinks" it focused at 10" I mean, you could check the EXIF for a close focus shot and read where it actually focused.

Cheers

James

My original post was trying to get at a simple point. Many here have pointed out the workng distance of macro lenses. I was trying to determine the real underwater working distance of the 60mm vs the 100mm Canon lenses. It seemed if the 1:1 distance on the 60mm was really 10" or more underwater, that would be perfect for my shooting, especially after working at 2" with the C-5050.

Something I've never tested but have wondered about. Need to test this:

Does the 4/3 flat port magnification effect yield a 4:3 effective magnification or does it simply increase the working distance and leave the magnification alone.

The new Canon 60mm does 1:1. That means 1.6:1 apparent due to the crop.

Canon's press release says the EF-S 60mm focuses at 1:1 at a working distance of four inches from the subject.

A lot to respond to, which is posted below in the same order as above.

Karl,

I do not have an UW photo specific website.

However, if you follow this link, there are hypertext links to some UW pix of mine from Prince William Sound.

http://www.pwssc.gen.ak.us/shepard/docs/re...dal/diving.html

They are small pix, which is good given the relative slow connection between here and the outside world. I frequently tell people that Alaska is on the gravel road of the information superhighway! (we have a lot of gravel roads here). The vis was quite bad at the location, so they look better small.

ReefRoamer, CeeDave is quite right, think of it this way:

1:1 means that if you take a picture of a 30mm long nudibranch it will appear 30mm long in the image (film or sensor) plane of your camera. With a 24 x 36 mm (i.e., FF) you will be able to fit it lengthwise with 6 mm to spare. Because the Nikon DX format (for example) is 23.3 x 15.5 mm, a 30 mm will not fit lengthwise into the frame at 1:1.

At half life size it would appear 15mm long in the image plane and would be able to fit with room to spare in a DX frame. At one half life size on a FF camera there would be a lot of space around it. To make one print from each camera in such way that the nudibranch is the same size (in the print), you would have to blow up the FF shot by 1.5 times as much compared to the DX shot.

The smaller the sensor size the smaller the field covered at the same reproduction ratio. Shoot a coin at 1:1 on 35mm film and it appears large; on 4x5 inch film it appears small (at 1:1). BUT, if you laid the negatives (or transparencies etc.) side by side on a light table the coin images would be the same size and would be the same size as the coin itself. There would be just a lot of wasted film on the 4 x5.

James, The Exif info in Nikon View does not show the focused distance on D70 shots (just checked). My guess is that what you are seeing is the same distance that you can read off the lens barrel, which would be the apparent distance. The Rolleimarin housing focusing knob had two scales, one for apparent, and one for actual distance. I gather former was to for guess focusing (they made an external metal frame-finder for the housing) when not using the focusing screen while the later could be used with a measuring tape.

Bandit,

I gather that the 5050 is a fixed-lens camera. It may zoom to a shorter focal length to achieve macro-range focusing, which means very short working distances. The 1.33 mag means that you do not need to even focus as close as 1:1 as inscribed on a macro lens barrel to achieve 1:1, so would have more WD. Racked out to 1:1 (as on the lens barrel), mag is 1.33:1 at the same WD as 1:1 in air (this WD has to include the space inside the housing in front of the lens as well as the port thickness; so distance from front of the port will be a bit less).

Craig,

I think I just answered your questions (first question -see reply to Bandit; second, the reply to ReefRoamer). It is still 1:1, just the format (senor size) is smaller!!!!!!

By blow-up I mean crop!!

T

Craig,  

I think I just answered your questions (first question -see reply to Bandit; second, the reply to ReefRoamer). It is still 1:1, just the format (sensor size) is smaller!!!!!!

No, what I want to know is whether the magnification increases through the flat port or the minimum focus distance increases. You get a 4/3 magnification benefit from the flat port but that doesn't help if you have to focus further away. It would be easy to test underwater but it's a little cold for me right now.

I'm not an optical engineer but I would expect that the magnification in the water/flat port interface would be the same as adding a diopter to the lens. In effect the water is another lens. In this case, you would get both a decrease in minimum working distance and higher magnification at the original working distance.

You may be a newbie but you have it figured out. The water-port-air interface is like a diopter. When in the pool I find it easier to read my watch while submerged (with a mask on)! I use reading glasses now that I am older.

The min focus stays the same but because of the mag you get a bigger image, hence 1.33:1 with a macro lens capable of 1:1. For this reason it is easiest to consider a focal length mag of 1.33. A 100mm becomes a '133' mm lens.

Cheers!

Tom

Here is an illustration to help debunk the DX/35mm ratio issue. To the left is a DX size chip in the center and right are good old fashion 35mm slides, the critter remain the same size in both the DX chip and the 1:1 center slide, there is no magnification in this case, just a area coverage shortage in the case of the DX chip, the right 2:1 slide illustrate when you add extension tube, diopter or teleconverter to attain bigger magnification, in this case you are closer thus you have a bigger image on the area of your chip or film, I agree with CeeDave aka Chris, life would be simpler in degrees coverage.

The second illustration demonstrate image circle, the image circle of a lens is appropriated to the format it was designed for, it's ok to use a smaller chip because the circle is bigger, but the not the opposite, hence the reason lenses of the DX, EF-S etc cannot be used properly on camera with 35 film or bigger and full frame chip. Notice the critter size and how the area coverage is the only change, the focus distance distance remain the same, only the area covered is wider in the case of full frame 35mm film.

Thanks for posting this Jean. I'm going to try to explain this to some friends at a pool session tomorrow.

Cheers

James