DOF problems

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hi, i have a canon rebel xti and i've started using the sigma 18-200mm f/3.5-6.3 lens.
i've tried taking photos with a low fstop number to produce a shallow DOF. but it hasn't been working. the background is not blurred as i had hoped. i don't understand what's wrong.
here are some examples of the photos i took recently with their respective fstop and shutterspeeds

1) http://www.flickr.com/photos/rebeccathecurious/424669627/
f/3.5, 1/1600 sec

2) http://www.flickr.com/photos/rebeccathecurious/424636546/
f/3.5, 1/3200 sec

any ideas as to what i'm doing wrong would be appreciated. thanks!
 
Every lens will produce a slightly different blurr, If you get to far away from your subject the camera will focus to infinity and the background will be less blurred. Try a couple of macro, close up shots with the same F/stop and see what happens.
Cosmo
 
f/6.3 on the long end is a pretty narrow max aperture. I'm thinking that you won't be able to get a shallow DOF out of that lens. One of the drawbacks to an inexpensive consumer zoom; especially one with such a long range. If you want to take short DOF shots, you might want to look at getting a 50mm/f1.8. They don't go for much.
 
thanks for the reply cosmonaut, but i have tried some close ups and they're a little better, but not considerably so.

i just find it really strange that the other shots have absolutely no blur even when i use aperture priority mode with the lowest possible fstop in excellent lighting conditions.
 
markc: thanks for the help, and i understand what you're saying, but the shots i took were only at 18-60mm zoom and yet they still did not produce a shallow DOF. i've seen other peoples' photos using the same lens and they're able to produce a shallow DOF even at the 100mm length. so i'm still quite confused, but thanks for the input.

and i already have the canon 50mm f/1.8 lens you're referring to. it's great, but i also need a lens with zoom capabilities. do you have any suggestions for better lenses that can produce shallow DOF's but have zoom? thanks
 
What is the max f# at 100? I'm guessing around f/4.5 or f5. You'll get some blur, but not much. Do you have a link to the shots you saw, or were they prints?

Also, the f# isn't absolute, it's a ratio to the focal length. F/3.5 will give you more DOF at 50mm than at 200mm, so even though the f-numbers are smaller at the wide-angle end than the telephoto on that lens, you are probably getting more DOF.

This is math, so you might want to ignore it, but f/3.5 @ 18mm is an aperture opening of only 5.14mm (18mm/3.5). That's the same as f39 @ 200mm. Most of the image is going to be relatively sharp @ 18mm no matter where you focus.

To maximize DOF, I would use the lens at 200mm and get as close to the subject as possible while having the background as far away as possible. It won't be great, but that's the trade-off between zooms and prime lenses. With a prime, you only have one focal length, but you can get a very wide aperture.
 
Oh, as far as a recommendation, I'd say anything with a larger max aperture. Unfortunately you have to start paying for that on a zoom. Frustrating, I know. There are ones out there that are better than 6.3, but not by much. 5.6 is common for consumer lenses, and you have to pay to get down to f/2.8. The main thing is to stay on the telephoto side of whatever lens you are using if you want shallow DOF.
 
oh. i seeeeeee. that's been very helpful. thanks, mark. i wasn't aware that the focal length divided by the fstop equals the diameter of the aperture (i'm very new to photography).

the shots i looked at were prints, but i've found some other examples:
1) http://www.flickr.com/photo_exif.gne?id=425886086
is this because the focal length is 140 and fstop 10, therefore 14mm aperture diameter, which is wide, so shallow DOF?

2) http://www.flickr.com/photo_exif.gne?id=409591014&context=pool-90562530@N00
tamron equivalent of the same lens. this shot uses the 200mm like you recommended.

ok, it makes sense to me now. but now i'm sad :( lol. i guess i'll just have to carry around both the zoom lens and the 50mm prime. thanks again for the help.
 
The wider aperture will surely help you to get a shallower DOF....especially at 150mm.

This is a fairly new lens, and I haven't heard much about it...however, it's probably fairly similar to the Sigma 70-200 F2.8, which is a pretty good lens.
 
thanks for the input, mike.

do either of you know whether the 50-150mm f/2.8 lens can also produce a shallow DOF at 50mm?

also, is the aperture fixed at 2.8 all the time on this lens or can the aperture be made narrower as well? that might be a stupid question, but i just want to make sure, lol. i'm very new to photography.
 
is the aperture fixed at 2.8 all the time on this lens or can the aperture be made narrower as well? that might be a stupid question, but i just want to make sure, lol. i'm very new to photography.
It's not fixed, F2.8 is the maximum.

do either of you know whether the 50-150mm f/2.8 lens can also produce a shallow DOF at 50mm?
DOF is a result of focal length, aperture and the distance at which the lens is focused. You will be able to get a more shallow DOF at 50mm with this lens, than you could with that 18-200mm lens...because with this lens, you can use an aperture of F2.8.
 
hi, i have a canon rebel xti and i've started using the sigma 18-200mm f/3.5-6.3 lens.
i've tried taking photos with a low fstop number to produce a shallow DOF. but it hasn't been working. the background is not blurred as i had hoped. i don't understand what's wrong.
here are some examples of the photos i took recently with their respective fstop and shutterspeeds

1) http://www.flickr.com/photos/rebeccathecurious/424669627/
f/3.5, 1/1600 sec

2) http://www.flickr.com/photos/rebeccathecurious/424636546/
f/3.5, 1/3200 sec

any ideas as to what i'm doing wrong would be appreciated. thanks!

Regardless, I like this picture a LOT:

http://www.flickr.com/photos/rebeccathecurious/424669627/

It's perfect, and I sort of like the fact I can read what's going on behind her.

Shallow DOF is interesting, but not always neccesary in my opinion. BTW I love your work, and your "whats in my bag" pic is great... gonna add you to my flickr friends! :)
 
Distance to subject plays a huge role, as well as focal length. If you are using that lens at the wide end, you won't get shallow depth of field. Frame the shot the same way, only at a focal length of 100 or more, with the widest possible aperture, and you'll get what you are looking for. It's not going to be anything like using a 2.8 lens however.
 

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