DOF

I'm getting more and more puzzled about DOF. I have just bought a 70 LTD and very nice it is too, no complaints there but I took a rather dull shot of which a 100% crop is as below.




What I don't get is

70 mm lens at F2.8

The focus was on the bricks
The sign is out of focus and it's a street sign mounted on the standard 2" tube being another 2" from the wall. Surely at over 6 feet (as I recall 8 feet) away the sign should be sharper according to DOF tables?

Or, as is usually the case have I misunderstood something?

Mike

Telephoto lens. Open aperture. Close distance. All this equates to shallow DOF, which is clearly shown in your image.

You focused on the bricks and the bricks are sharp.

Focal length (mm) 70mm
Selected f-stop f/2.8
Subject distance 8 ft

Depth of field
Near limit 7.79 ft
Far limit 8.22 ft
Total 0.44 ft

In front of subject 0.21 ft (49%)
Behind subject 0.22 ft (51%)

Hyperfocal distance 284.4 ft
Circle of confusion 0.02 mm

if you go to 6 feet
Subject distance 6 ft

Depth of field
Near limit 5.88 ft
Far limit 6.12 ft
Total 0.24 ft

In front of subject 0.12 ft (49%)
Behind subject 0.12 ft (51%)

Hyperfocal distance 284.4 ft
Circle of confusion 0.02 mm

as you can see it's already the half of the DOF so if it was just a little less then 8 feet it's out of focus.

Use this online DOF Calculator

http://www.dofmaster.com/dofjs.html

shim

So basically you guys are saying I'm an optimist and that DOF tables reflect in the image?

Mike

it's really on the edge of the DOF so it might be a little back focused maybe since the wood on the back is still fairly sharp.

Assuming DOFMaster's numbers quoted by Anvh are right, the DOF for 70mm at f/2.8 and 8 feet is +/- 2.4 inches approximately. You guestimated the sign to be 4 inches from the wall and so it would be a little out of focus.

Interestingly, the figures for the 70mm at f/2.8 are the same for 50mm at f/1.4.

Apart from the DOF issue, how are you finding the lens?

Best wishes, Kris.

Kris I guess I didn't convert the DOF figures right when I used the tables from an on line calculator. I used to know all these things years ago!

I have barely used the 70 really but so far I like it. It's what a lens should be. Compact, light, easy to use, it's sharp, the colours are good and overall as you'd expect it seems a good lens. How much better than the Tamron 17-50 which is my usual walkabout I can't say yet. But I do like the fact I can have the 70 on the body, the 35 in one pocket and now thanks to Clarky the 21 Ltd in the other pocket. No bags!

It is beautifully made and I love the cap and hood which I know irritates some people. And from SRS it seems cheap really.

Mike

I saw the 70mm Ltd at SRS last Saturday. I had no idea it was that tiny. Stunning bit of design.

K.

I am certainly enjoying my three primes as they are so very compact. I have the 21 ltd, the 70 ltd which is as you say very compact - did you see how the hood pulls out and it's still compact? I am still struggling to get the hang of focussing in some situations. The final lens is the 35mm FA which is not so small but is F2.

No more bag for me. Just one on the camera and one in each pocket so holidays will be much more pleasant. Well I hope it works that way.

Mike

Sounds like a nice solution. If there was an MX-sized Pentax dSLR I would be tempted to go down that route too, although I am infamous for carrying big bags of stuff (not just camera equipment).

I chose the 43mm Ltd because I want to be able to use them on my film cameras too. I'll have to win the lottery before I can get the 31mm and 77mm Ltds.

Cheers, Kris.

Quite. I looked at the full frame ltds and quickly realised they were out of my league. I only got the 70 as a reward for a very, very hard month's work and the 21 because that devil Clarky offered such a tempting deal!

I find the 21 quite wide but pleasant to use and I'm sure when the better weather comes that's the one that will live on the body. I've put a recent 21 shot into the Your Photos bit (well I will have done is two or three minutes).

Mike