Newbie Question

[The23rdman]

Can someone please explain what the numbers on the very front of my lenses mean, please? By that I mean the numbers in feet and meters and the infinity sign. Also how do I interpret and use them?

Thank you.

 

[HoppyUK]

It sounds like you are referring to focusing distances, but those numbers are not on the "very front" of the lens. The ones on the front are the lens specification - focal length and maximum aperture (when focused on infinity). Often the filter thread size, too.

 

[The23rdman]

It sounds like you are referring to focusing distances, but those numbers are not on the "very front" of the lens. The ones on the front are the lens specification - focal length and maximum aperture (when focused on infinity). Often the filter thread size, too.

Yes, the focusing distances is what I'm referring too. How do I interpret and use them?

 

[HoppyUK]

Yes, the focusing distances is what I'm referring too. How do I interpret and use them?

They tell you what distance away the subject is that the lens is focused on, and are measured from the sensor forward.

They are most relevant when calculating depth of field, which is the zone of acceptably sharp focus both in front of the main focus point and also behind it.

Depth of field (not to be confused with depth of focus, or field of view ) varies according to the focusing distance, the lens focal length, and the lens aperture (f/number) you are using. (It also varies with the size of camera sensor, but you can't change that without buying a new camera.)

If you input those details into a depth of field calculator like this www.dofmaster.com you will see exactly how it changes.

Edit: DoF calculators take focusing distance from the front of the lens, not the sensor. Doesn't make any difference normally, but can be a factor in close-ups. Bear in mind that depth of field is not an exact science, even though it is presented like that in DoF tables and calculators. It is only exact when measured according to very strictly defined parameters. In practice, sharpness in a photo gradually fades from being very sharp, to quite sharp, to not quite so sharp, to blurred. The point at which it becomes unacceptably sharp to you varies a bit, and it is often quite a bit more if you only do small prints, and a bit less if you make big prints around A3 size for example.

 

[The23rdman]

That's great, thank you. The problem I've been having is calculating where to focus when composing a story-telling scene.

 

[HoppyUK]

That's great, thank you. The problem I've been having is calculating where to focus when composing a story-telling scene.

Sounds like you have been reading that book Understanding Exposure by Bryan Petersen I think his imaginative use of language is not always helpful, but I know you have already seen this thread, but here's the link anyway http://www.talkphotography.co.uk/forums/showthread.php?t=153343

The easiest way to focus when composing a picture, I think, is to use the centre AF point and aim it at the main subject of interest, lock it with a half-press and hold it there while you reframe the picture how you want it (usually not with the main subject smack in the middle).

That will work if you have one main subject, and all of it is at pretty much the same distance from the camera. But quite often the subject is larger and longer, or you want both the main subject and the surrounding area sharp also, perhaps a landscape where you want everything sharp right from near the camera to the far distance (infinity point).

In this case, you would measure the near and far extremes of distance needed to be sharp, consult you DoF tables, select the appropriate f/number and then focus on the point indicated in the table, usually a mid-way position some way into the scene.

Easier ways are to just estimate, focus half way in, use a high f/number, and see what it looks like on the LCD. This might sound slap dash, but with a bit of experience, this usually works just fine. I often work like this, and if I'm in any doubt, I just use one full f/number higher to make sure. It's fine. Depth of field doesn't have to be difficult and confusing.

Your Canon 350D also has an A-DEP mode, and that works just like consulting complex depth of field tables. You aim the camera at your main subject and the camera will set the f/number (and shutter speed and focus distance) needed to ensure that all areas are sharp if they are covered by the flashing AF points in the viewfinder. You can change this by moving the camera so that you get the important parts of the image covered by flashing AF points, then hold and lock that position with a half-press and reframe the picture as you want.

 

[The23rdman]

That's a brilliant post, Hoppy, thank you. I've basically been doing what you suggested in that on landscape type shots I've been choosing a nice high f/number and just kinda going for an area 1/3 to 1/2 way into the shot having either focused manually or the way you suggest.

I have used the A-DEP mode too, but found it limiting at times. I'm the kind of person that needs to understand what I'm doing to feel like I'm making progress so the links you gave to the calculator will be a really useful tool for me. Thanks again for being such a help.

 

[HoppyUK]

That's a brilliant post, Hoppy, thank you. I've basically been doing what you suggested in that on landscape type shots I've been choosing a nice high f/number and just kinda going for an area 1/3 to 1/2 way into the shot having either focused manually or the way you suggest.

I have used the A-DEP mode too, but found it limiting at times. I'm the kind of person that needs to understand what I'm doing to feel like I'm making progress so the links you gave to the calculator will be a really useful tool for me. Thanks again for being such a help.

No worries bud, my pleasure

If you're in learning mode, try printing out this handy (and free) rotary depth of field calculator. You specify your focal length and customise it to whatever distance and f/number scales you want, in feet or metres. Specify the size, cut it out and stick it on your lens cap. You can even do several focal lengths on one dial, to cover your zoom range

It's so easy, and you will instantly grasp what is going on as you change focusing distance, and focal length. I remember my dad teaching me about depth of field as a lad and I couldn't get it at all. But I was looking for something way too complicated and when the penny dropped I just thought so that's all it is!

Anyway, go to this page of dofmaster.com http://www.dofmaster.com/custom.html and download the PDF template, it's a really neat little programme.

When you've got to grips with it all, you will find that depth of field is affected by two key factors - image magnification, and f/number. Magnification is a combination of focal length and focusing distance. Fit a longer lens with double the focal length, image gets bigger, depth of field reduces. By the same token, move closer to half the distance, image gets bigger, and depth of field again reduces by the same amount. People say that telephotos give less depth of field and wide angle lenses give more, but that isn't true. They just make the image bigger or smaller if you don't change your position. But if the image size remains the same in the viewfinder, depth of field also stays the same regardless of the lens.

Anyway, take a look. You can learn a heck of a lot without printing anything out, just by rotating the two dials on screen and seeing the zone of depth of field that results.