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Help with using lenses and apertures


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For those of us who are not expert photographers it would have been nice to get some more guidance during the training class about when to switch between various lenses(e.g. between 50mm and 100mm lenses) and when to change aperture settings. Another way to say this is how do lenses and aperture settings change as you get close up or far away and as things get larger and smaller?

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hi Jerry,



The 100mm lens and the 50mm lens are great lens selections because they have really well developed optics and very little lens distortion. These two lenses always go into our camera bag.


You can use the 100mm lens when you want to 'get further' away from your subject, which creates a 'work' area between you and the object. Taking this step will reduce the risk of knocking into your tripod, introducing movement (bad). When a 100mm lens is put on a CMOS sensor that has a crop factor of 1.6, then that lens effectively becomes a 160mm lens. This would back you out even farther from your subject.


Numerous Canon cameras, such as the Rebel products, have the 1.6 crop factor. This can be great when you want to shoot macro, more reach. Its a give and take-trade off. When you place a 100mm and 50mm on a Full Frame camera, (like the 5D markII), the 100mm lens is truly a 100mm.



The 50mm lens is a good choice when your subject is physically bigger, and you simply need to get the subject into the frame. Or maybe you're shooting in a narrow hallway, and you're forced to use a shorter focal distance.


Aperture Settings: when do you change them?

this could be a really really long reply, but the readers digest version is that (in re: to RTI) you want you subject and spheres in focus and you have to adjust your aperture settings to get that focus. General rule of thumb is that F8 to F11 is a sweet spot. If you can get your exposure settings into that zone you're on the right path.


For RTI, start your settings at F8 @ 1/125th @ 100 iso then 'adjust to taste'. Always try to capture at iso 100. Don't go above your flash synch speed (no more than 1/200th) and then adjust your F-Stop.


Rule of thumb with Aperture is the bigger the number (F-22) the smaller the hole, the smaller the number (F2.8) the bigger the hole. Bigger hole, means more light gets in, smaller hole means less light gets in.


If you're using a small aperture, (big number/small hole) then you're going to need more light - increase Flash output)

If you're using a big aperture, (small number/big hole)(also known as shooting 'open') you're going to 'reduce' the light output.


I hope that this helps you and helps to shed some light!


Here's some home work reading :



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  • 2 months later...

Hi Marlin! How are you? I'd also love to get a bit more info on the lenses, please.

I finally have time to start looking at lenses and I was thinking about getting a 100mm and a 50mm. But, with the 1.6 crop factor in my Canon Rebel, a 50mm lens wouldn't be a true 50mm lens either (it becomes an 80mm lens), correct? So is it still recommended to have a 50mm lens or should one count the crop factor and use a 35mm instead? (Or would there be some distortion with a 35mm?)



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  • 2 weeks later...

hi Ana,


hope this message isn't too late, anyhow ...



yup, the 1.6 crop factor is an issue. But sometimes is an advantage. The case of shooting a macro, its fabulous, bc you get closer to the image. The 1.6 can also give you more 'work space' when you need to get farther away from the object/artwork. (think work space for string, moving around, etc)


The 100mm Macro (2 versions)

01) This is a great lens that we used for years. good optics, workhorse!

http://www.bhphotovideo.com/c/product/1 ... Macro.html

02) This lens is *awesome* - BETTER optics - and more $$$

http://www.bhphotovideo.com/c/product/6 ... Macro.html

-make sure that IS is *OFF - (at least for RTI)


50mm Macro

http://www.bhphotovideo.com/c/product/1 ... Macro.html

-this is our other workhorse.

-this lens will be just fine on the 1.6


1.6 & 35mm

in re: to the 35mm on a 1.6 cmos sensor. This question is a slippery slope. Every camera lens has lens distortion. Some lenses have a lot, some - hardly any. We try and stick to the 'hardly any' group of optics. If you purchased a very high quality 35mm and put it on a 1.6 you'll get less distortion than on a cheap 35mm lens - but you'll still get some sort of distortion. Try and stick to the best quality glass with the least amount of lens distortion - thats the best advice i can give.


If you have to shoot with a lens that has distortion (bc your bag got stolen, bc it got dropped, bc it too a swim) shoot with what you've got. Get the best data that you can. Be sure to take as many notes as possible about the setup and plan for some Distortion Correction Lens processes in post production. We didn't really get into it in the RTI Training, but there are ways a photographer can adjust for lens distortion, but thats a different post. And oh, shooting a photogrammetry data capture will also rectify the lens distortion issue.


hope that this helps.



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  • 2 months later...

Just wanted to throw a bit more into the mix here. Like most things in life there are trade-offs.


The 50mm macro is going to have more depth of field than the 100mm macro. Also, pulling back from the subject can give you more depth of field though you will lose some pixels on the subject - sometimes that's what it takes to get the important material in focus.


As for aperture, Marlin hinted at this, but I want to be specific. There is a "sweet spot" for each lens of the aperture that will provide the best sharpness. There are tables of these things you can look up for your particular lens. For the 2 lenses we are talking about here (both Canon Macro's) that is at 6.3 - 7.1. As you change to a smaller aperture, you lose sharpness. This is a function of the diffraction of light through the lens (see links below for more info) So while f-22 gives you more depth of field, you lose sharpness, and the ability to resolve definition (i.e. line pairs per millimeter) So, stopping down to gain depth of field can be your enemy because of this. We generally do not go above f11 on these lenses, because of the loss of sharpness. You can test this yourself easily enough or just follow the links below.


Here's a great article about diffraction and how it affects lens sharpness: http://www.kenrockwe...diffraction.htm

Here's a photo example that demonstrates it well: http://www.kenrockwe...son/f-stops.htm



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Thanks for this discussion.


I will be documenting rock art outside with a Nikon D90 and have a 18-200mm Sigma zoom lens. I am afraid I may have trouble with vignetting. Any thoughts on this?


I appreciate any guidance.

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hi Ansley,



I wouldn't necessarily worry about vignetting unless that particular lens is partial to that behavior with the cmos sensor 1.5 crop factor. You should consider setting up that camera and that lens, shooting some test images to verify any vignetting. Thats the only way to really confirm any errors you might be creating. If you dont see any at 18-100mm you're in the clear — for vignetting that is ... but that lens will introduce other unwanted behaviors as described below ...


Be careful of Lens Creep.

This occurs mostly when the camera is pointed "down", frequently your lens might "move" and change your focus bc of the physical weight of the lens "hanging" down. The first few shots of your sequence might be OK, but then, towards the end of your capture, the lens could "creep" closer to the earth — introducing focus errors. If you have to use that lens, use simple "tape" to tape the lens in place.


Also consider the amount of lens distortion that goes along with using a telephoto lens. Sigma lenses, typically have more distortion than other lenses. If you have $ in the budget, stick with a "Prime" lens, which has a LOT less distortion and zero chance of "lens creep". Be sure to check out "www.dpreview.com" -- look up your lens --- then click on its properties to discover the actual amount of distortion in that glass. Check out the nikon 65mm and the 105mm macro lens — both prime, both quality, both expensive.


Also consider the 1.5 crop factor of your camera. If you're zoomed in to 100mm, the actual focal length will become 150mm. Questions to ask you self now, is how much distance do I need to between the subject and the camera. On the flip side, this could work to you advantage if you're shooting a macro object.


In a nut shell, you can shoot an RTI with any lens, its just a trade off when you do ... in general, really try to spend your money on good high quality lenses. Good glass will last a long time.



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  • 2 months later...

Marlin --


I've noted that f8-f11 is the sweet spot. Logically, though, it seems to me that you'd want as tiny an aperture as you can get to minimize focus problems (maximize depth of field). What, in your experience, is the problem with using ƒ32, for example?


--Mike Jennings


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  • 3 weeks later...

Can you give me some input on setting the level of the flash output? For example, if I am shooting indoors with little ambient light, (like in your studio or darker) starting with the settings Carla suggests f8.0, iso 100, 1/125 sec., what flash output is best? Is there a "sweet spot"? I understand increasing/reducing the flash output depending on aperture size, but I am not sure what I should be going for.

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Hi Lori,


The light power is about getting a proper exposure and that will depend on the size of the object you are shooting and how far away the light is (the string length) and also what kind of light source you are using. So, there isn't really a rule of thumb here, and you need to experiment a bit. Here's an example, with the Canon Speedlite 580exII flash units, we might use 1/4 power for shooting an object area with a 32in string length. Then you need to look at the histogram of a high and low angle shot and adjust accordingly.

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Can you tell me how to get the flashes in the blended sphere to be more precise? I get a good distribution but the flashes aren’t as clean as I know they should be. I reduce my movement and the flash output with some success, but I still am not replicating what we got in class. I think it must have to due with light intensity. Most probable fix?

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Lori, a few thoughts about possible fixes:

  • Check the aim and distance of the flash. I've found that the flash coverage on the object and reflections on the spheres seem to be more sensitive to the aim and distance when working close-up, if that's what you're doing.
  • Check the settings on the 580 EXII flash, especially the zoom. Set it for as wide-angle as possible (I think this is 24mm on the 580EXII). Make sure the mode is set to manual.
  • Make sure the spheres are clean. Smudges and scratches can significantly affect the sharpness of reflections.
  • You might need more depth of field if you're working close-up. The minimum aperture I used was f/11 to get more depth of field. The trade-off with smaller apertures (higher f-numbers) is losing sharpness (not the same thing as focus), as Marlin and Carla have said above. Mostly, I used an aperture of f/5.6 and I was willing to accept a very slight blurriness of reflections on the spheres. I gave priority to getting a sharp focus on the object surface and using a wider aperture (smaller f-number), as long as the reflections were reasonably small, sharp, and well defined (*caution--unofficial advice*).
  • Are the spheres you're using larger than you need? They should be a minimum of 200 pixels in diameter, but if they're more than about 300 pixels across, you could just be wasting real estate. If they're too large, or if the spheres have to be placed above the surface of the object, you will need more depth of field to keep them in focus. Consider choosing smaller spheres, if possible.

Sorry if any of the above is redundant with what Carla and Marlin have said, or if it reiterates things you've already tried. Good luck!

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