macro photography: better focal length?

[jamesiwalker]

I recently got a Canon Digital Rebel XT, a 50mm f/1.8 lens and
an extension tube set - 12mm, 20mm, and 36mm . I am still not
able to find how to get a good focal length using what I have.
The focal length is really narrow, basically most
flowers will be blurred at least in one spot when I take a photo,
unless the flower is really flat. How can I get quality focus and
close up? I can afford another lens but dont know if I need one -
any suggestions with working with what I have or suggestions on
if I should get another lens?

 

[xfloggingkylex]

Well I'm only a nooby and am not sure what the tubes do, you could probably just stop down a little less to say f/2 or f/2.8 to give a slightly deeper depth of field (DOF) so that the entire flower is in focus.

Someone who knows better will be able to further help you.

 

[Big Mike]

When you say the focal length is really narrow...I think what you mean to say is that the DOF (Depth of Field is really narrow). The focal length is 50mm (lens) plus whatever extension tubes you have on.

When doing macro photography...the DOF will be very thin...that's just the physics of it. To maximize the DOF you will want to stop down the lens. (use a smaller aperture). A small aperture would be F16. You could even go down to F22.

When you stop down that much (small aperture) you will need a fairly long shutter speed...most likely too long to hand hold...so you will need a tripod. If you have a remote shutter release...use it, otherwise use the self timer. If the XT has MLU (mirror lock up), use that too.

 

[Rob]

I believe that a macro lens would give a slightly greater DOF for a given focal length at a given magnification... however, I could easily be wrong there!

I was having a play with one of these the other day, and it's pretty good VFM considering the quality: http://www.bhphotovideo.com/bnh/con...&Q=&sku=194451&is=GREY&addedTroughType=search

Rob

 

[jamesiwalker]

This is confusing me. My mother has an Olympus Camedia 5000 (not
an SLR) and I've gotten better macro shots with decent depth of field
than on my Canon Rebel XT, and mine is suppossed to be a better
camera. With her camera, I didn't have to worry about long shutter
speeds and shallow depth of field when using the super macro.
Could someone explain this?

 

[Torus34]

Macro lens: a lens which will permit you to focus on subjects much closer to the lens than non-macro lenses. There is no formal definition of 'macro' any more than there is a definition of 'high fidelity.'

Close-up lens: a lens placed on the front of a lens to permit focussing on subjects closer to the lens than otherwise possible. Supplied in diopter strengths of 1, 2, 3, 4 and 10. When stacked, strengths are additive. There can be a small loss of definition.

Extension tubes: let you increase the distance between a lens and the film plane. This in turn permits you to focus on subjects much closer to the lens than when the lens is mounted directly on the camera. Tubes normally come in a set of different lengths. The tubes can be stacked. Some expensive sets permit the camera to be operated in automatic mode [though this is not very important.]

Reversing ring. When you wish to get so close to a subject that the size of the image on the film will be larger than the size of the subject itself ['magnification' of the subject], a reversing ring is used to 'flip' the lens around. This results in a sharper image at the point of focus. [Lenses are not formulated to run in greater than 1:1 magnification.] Reversing rings are not usually 'automatic.' You must go to manual mode. Images larger than 1:1 will require an increase in exposure.

Depth of Focus [DOF]: the range of distances within which things will appear to be in sharp focus. [Actually, only one specific distance will be in sharp focus, but the degree of 'out-of-focus' of objects within the DOF will not be noticeable.] Objects closer or further away than the limits of the DOF will be noticeably out of focus. Those closest or furthest away will be out of focus by the greatest amount. DOF is a function of the focal length of the lens*, the distance to the subject and the f stop used for the exposure.

Shorter focal length of the lens = greater DOF. Higher f stop number = greater DOF. The further away the point of focus is from the front of the lens, the greater the DOF. The precise distance on which the lens is focussed is 1/3 'deep' inside the DOF.

For the best results in extension tube/reversing ring/close-up lens 'macro' photography a tripod, a hand-held exposure meter, a cable release [use 2 or 10 second delay on digital cameras], manual focus, manual exposure and the smallest possible f stop are usually 'standard.' In general, 'macro' work is a slow process due to the accurate set-up required.

NOTE: Focus with the lens wide open. Then stop down as far as possible [highest f number] for the actual exposure to provide the greatest DOF. Remember that the DOF is 1/3 in front of and 2/3 to the rear of your point of focus. As you stop down, correct the exposure by decreasing the shutter speed.

NOTE: The older manual focus 35mm rigs may be easier to use in 'macro' work than today's digitals. The cost of film is rarely a problem due to the deliberate nature of 'macro' work.

* Probably answers your last question.

 

[jamesiwalker]

thanks, got better depth of field now.
just had to move further away from subject.

 

[Big Mike]

This is confusing me. My mother has an Olympus Camedia 5000 (not
an SLR) and I've gotten better macro shots with decent depth of field
than on my Canon Rebel XT, and mine is suppossed to be a better
camera. With her camera, I didn't have to worry about long shutter
speeds and shallow depth of field when using the super macro.
Could someone explain this?

One of the determining factor in DOF is the size of the recording medium (film or sensor). Smaller sensors (like in your Mom's Olympus) make for larger DOF. For amature macro photography...digicams are quite often much better than a DSLR. Because the DOF is deeper, the digicam doesn't need to stop down as much and therefore doesn't need such a long exposure time.

On the other hand, I see lots of of questions about how to get a very shallow DOF with a digicam...and and the answer is often...'you can't'

Don't worry, your Rebel XT is much, much better than any digicam...it just doesn't necessarily do macro as well as they do.

If you wanted to get into high end macro...with proper lens, lights, meters etc....then your camera would give you better results.