Boo's Digital Photography Tutorials.  ~ No. 11

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Depth of Field and Hyperfocal Distance

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Depth of Field; sounds scary:

If there's one single aspect of photography that catches out new photographers more than any other, it's depth of field - or more often, lack of it.

Depth of field (DOF) is the amount of a scene - from front to back - when captured in an image, that is an acceptable level of sharpness.

 [Shallow DOF can also be attractive if you focus with care. ] Is it always the same? No. There's a complex geometry at work and the finer technical mathematical points of it are beyond the scope of this article, but I'll explain enough of the fundamentals ~ hopefully in simple and practical terms ~ to at least give you a basic understanding to apply in the field when taking pictures. I've stretched some of the mathematics a little to make the generalisations easier to remember. Accuracy is only worthwhile if you can actually remember it. In this instance, I've erred towards practicality over strict accuracy.

Please note: These tutorials are written specifically for users of crop format sensor DSLRs, in my case the 300D and 20D and any specifics or numbers given relate only to this range of cameras with the same relative sensor size. Whilst the principles can be applied to any camera, specific numbers and figures may not be appropriate to other models - you will need to make adjustments for the relative crop ratio of your own sensor. Please see this on-line DOF calculator which allows you to enter your camera model and choose apertures and focus distances in order to calculate DOF.

In the case of DOF specifically, sensor size is a very pertinent factor, the smaller your film plane or sensor is, such as with compact digicams, the greater the DOF for the same composition, focus distance etc. So please bear this in mind when reading this page, as it was written with x1.6 crop factor DSLRs in mind specifically. Whilst the principles are the same for all cameras, the specifics are not.

Regardless of the camera used, if the DOF is too shallow for your scene, in that not enough of it is sharp, it can not only spoil the perceived success of the photograph, by giving the impression that it isn't as sharp as it might be, it also minimises the margin for error for both photographer and subject movement while composing and framing the shot, especially problematic if, like me, you like to choose your own point of focus, then re-frame your image. The slightest wobble of subject or camera, changing the distance between them, can prevent an image being in focus, or cause it not to be in focus where you expect or hope.

 [For most landscapes an aperture of f11 will provide sufficient DOF. ] The amount of DOF you have available is determined by a combination of factors and changing any of them will impact on the results, so once you understand the basic principles, if you feel the DOF will be insufficient for the result you want, you can manipulate the factor that's most flexible in your scene to improve the result.

Don't think for one minute that this mysterious DOF factor is all doom and gloom - far from it, it's an amazingly powerful creative force, but you need to understand it, the factors that control it and once you're the one in control, you'll have a great deal of fun making it work for you as a creative element in your photographs. It will also impact on the quality of your results, allowing you to use the best aperture for sharpness, yet maintaining sufficient DOF for a good shot.

How do I know how much I have?

The amount of DOF you have is determined through geometry and the salient triangle of factors are your focal length, aperture and your distance from the subject. The basic principles that govern the DOF in your resulting image are:

Aperture: a smaller aperture = greater DOF.
The smaller the aperture of the lens, the greater your available DOF. A smaller aperture - a smaller hole in the iris - but a larger number - will increase DOF. For most general purpose shots, apertures in the range f8 - f11 will give the optimal sharpness from the lens with maximum DOF. Unless you're very close to your subject and using a very long focal length, it's unlikely that going much tighter than f11 will offer any significant advantage for most scenes.

My favoured optimum aperture for landscape scenes would be f 8 or f11 - unless you have elements very close to you that you want in focus, f11 will give reliable landscape scenes on the vast majority of occasions, there are few scenes where you'd actually need anything tighter than this. I have a little hyperfocal rule I apply in such scenes, which I'll outline below.
Focal length: longer focal length = less DOF.
The more you 'zoom in' on your subject, by increasing your focal length, the more you compress your DOF. If you shorten your focal length by using a wider angle, your depth of field will then increase.
Subject distance: the closer your subject, the shallower the depth of field.
 [With close shots and longer focal lengths, your DOF can be miniscule.] This is why macro and close up photography is so problematic, you're often working with long focal lengths and very lose to the subject - one of the features a macro lens allows - and therefore your available depth of field can be miniscule, literally a millimeter or less for very high magnification shots. These are probably the only set of circumstances where stopping your lens down to very tight apertures is justified. For most general purpose photography, you shouldn't really need to stop down further than f11 or f16, if you understand depth of field and frame your scene and choose your focus point carefully.

To summarise the factors that affect Depth of Field (DOF):

Less DOF:
Less of the subject, from front to back, is of an acceptable level of sharpness
More DOF:
More of the subject, from front to back, is of an acceptable level of sharpness
Wider aperture, opened up (smaller number). E.g. f1.4, f2.8 etc. Tighter aperture, stopped down (bigger number). E.g. f11, f16 etc.
Longer focal length, more zoom (e.g. 100mm, 200mm etc.) Shorter focal length, wider angle (e.g. 18mm, 28mm etc.)
Closer to subject, close focus point Further from subject, or distant focus point
Example scenario: Wide open lens aperture, zoomed in and close to the subject, such as macro/close up shots in low light. Example scenario: Stopped down, tight aperture, wide angle, landscape shots.

How do I know where my DOF is?

 [Shallow DOF can be effectively used to isolate your subject. Focus part way through your subject, not at the front.] One of the common mistakes that catches people out is where their DOF occurs within the image. Many people mistakenly think that if they focus on the foremost point of the subject they want in focus, everything behind it will be sharp. But your DOF extends both in front of the point of focus and behind it too. Using this information to your advantage can maximise on the success of focus in your shots.

It can vary from almost half in front of the point of focus and the balance behind, to the vast majority of it behind, as in the case of landscapes taken with short focal lengths. So if you know that you have a limited amount of DOF for your chosen settings and scene, choose your focus point carefully. See my tutorials on using the 50mm f1.8 lens for examples of how where you focus effects the perceived sharpness of shots and my 300D FAQ on using the DOF preview button for more details of how this can help you estimate available DOF.

A very common mistake with landscape photography especially, is that photographers think they need a very tight aperture in order to secure the massive DOF needed for landscape shots, but once you get beyond about f11, you encounter diffraction from the lens iris blades and the images themselves are consequently less sharp regardless of any other factors. So ensuring you get sufficient DOF for the scene, but using the sharpest and sweetest part of the lens is a very worthwhile skill to hone and photographers are very often surprised to find that you can get good DOF in landscape shots, even with quite wide apertures. None of the landscapes on this page were taken tighter than f11 and most have foreground and details on the distant horizon, all sharp.

Hyperfocal Distance rule of thumb for landscapes:

 [Understanding hyperfocal distance will ensure everything in your landscapes is sharp. ] I have a quick DOF rule of thumb that I apply for landscapes, based on Hyperfocal Distance. Hyperfocal distance is a point in space in front of you that is determined (and can be calculated) by the aperture and focal length of the lens you're using - and if you focus at that point, everything from half way between you and it and then to infinity will be in focus. So, if for example, the lens aperture and focal length determines that the hyperfocal distance is 12 feet - and you focus at a point 12 feet away - everything from 6 feet from you, all the way to infinity will be sharp, in your resulting shot.

Hyperfocal distance is also the point in front of you for the focal length and aperture combination where if you focused at infinity, the near point of acceptable focus would be. So you can see how focusing at that point would give some sharpness in front of it, but also all the way to infinity. You can download various charts and there are many sites that allow you to work out the hyperfocal distance for any particular aperture and focal length combination - and there's a simple formula if you want to work it out yourself: H (in mm) = focal length ² / f stop x CoC (circle of confusion, for the 300D and 350XT, this is 0.019mm).

 [Foreground and horizon elements can all be sharp with careful planning. ] But my own quick rule of thumb that's easy to remember in the field is 11:30. That's all I need to remember - that at an aperture of f11 and a focal length of 30mm, hyperfocal distance is 11 feet (it's actually a bit more, but the easy to remember version works reliably enough).

So if you set your aperture at f11, focus on something 11 feet away at 30mm - or wider - and everything in the scene would then be sharp - as long as nothing is nearer to you than 6 feet. If your key scene elements are further away than that, you can open up even wider - f8 would work very well - right at the sweet spot for many lenses. The f8 rule of thumb I remember is 8:18 : At f8 and 18mm, hyperfocal distance is 8 feet away.

If you zoom in, you need to rethink:

Of course, as soon as you zoom in on the scene by increasing your focal length, your hyperfocal distance also zooms off into the distance and the success of the shots using this technique will be undermined if you have closer elements in the scene. For longer focal lengths, the approximate rule of thumb I remember is that in the f8/f11 aperture range for landscape shots, the hyperfocal distance very roughly approximates to focal length, so at 50mm, your hyperfocal distance is around 50 feet and at 70mm, it's very approximately 70 feet.

If you change aperture this then doesn't apply of course, but as a vague generalisation, if you can remember this much and the principles behind hyperfocal distance and depth of field, you can estimate in the field, the effect of changing apertures and focal length will have on the effective depth of field within your scene without resorting to a calculator or needing a hyperfocal chart, especially if you use a bit of trial and error with the scene and test the focus using my technique described below, before committing to the shot.

The basic principles of using hyperfocal distance to focus and determine available depth of field that you need to remember is that with wider angles/shorter focal lengths, the hyperfocal distance is closer to you and the longer the focal lengths and more zoomed, the further away the hyperfocal distance. Couple this with the effect of aperture changes in that wider apertures send the hyperfocal distance further away and a stopped down aperture brings it closer.  [Choosing your point of focus carefully will ensure that everything in the scene is sharp.] So if you want to utilise a hyperfocal distance closer to you in order to maintain focus on closer objects in your scene as well as distant ones, or maximise on depth of field in a scene, then wide angles and tight apertures are the way to go. If on the other hand, you want to take a landscape scene where all the areas of interest are very distant from you, you may well get sufficient DOF even with wide open apertures, if you're working with a wide angle and your areas of interest are not close to the camera.

How do you know if it will work?

 [Using the hyperfocal trick to pre-focus allows quick shots on the move. ] I have a trick I apply to tell if it will work for my shot before firing the shutter. I focus my scene using Autofocus at the point in front of me that I calculate to be the appropriate hyperfocal distance, then switch the lens to manual focus. If I then scan over the scene with my finger on the shutter button and touch it over elements in the scene at different distances - that I want in focus - if I get the red blink of the focus point in the view finder and the green focus lock spot in the display - I know those points will be in focus when I frame and take the shot. In the shot above right with the boat passing between the trees, I tried this over the foreground trees and the distant peak to ensure that there was sufficient DOF for both extremities to be within focus.

 [Using the hyperfocal trick to pre-focus allows quick shots on the move. ] Another associated trick I often use to take advantage of hyperfocal distance is to pre-focus the camera at a hyperfocal distance and aperture combination (in Av mode) that I know will give reliable results and then put the camera into manual focus - I do this often when going along through nice scenery in the car (as a passenger I hasten to add) when I often risk quick shots as we move along, through the open car window. If I set the aperture at f11 and pre-focus on a road sign or something I judge to be about 15 feet away - I know my focus will be good for any landscape shots in the 18-35mm focal length range without the need to wait for focus on the move. Both of the shots in this block were taken that way whilst in a moving car.

Using Hyperfocal Distance for panoramas

The hyperfocal distance trick above works especially well when applied to taking multiple landscape frames for stitched panorama shots, where consistency between frames is a key element to the success of joins and the panorama as a whole - and reliable and consistent focus is no exception to that. For my panoramas, I set everything manually, from exposure to white balance and focus - this ensures that each frame has been taken with the same settings and consistency between the frames is ensured. Panos that don't work well, or have very visible joins, are usually down to lack of control in the taking of individual frames.

See my tutorial on creating panoramas for more on my own technique for panorama scenes and preparing and stitching frames and examples of complete panoramas.

Use hyperfocal technique for successful panoramas.  Holehird Gardens in Cumbria.  Large file!

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