Category Archives: Technique

Field of view and 35mm equivalency

Another topic that frequently seems to confuse people, and for which they appear unable to engage in even a small amount of self education.

The focal length of a lens is the distance between its optical center and the point at which parallel rays of light are brought to a point, or focused. It is an optical characteristic of the lens and never changes regardless of the size or type of imaging sensor upon which the image is focused (film, digital). This isn’t a matter of interpretation or opinion, it’s physics.

Lenses produce a circular image, no surprise given that lens elements are circular. The brightness of the image falls off towards the edges of the circle when the lens is wide open, the effect diminishes as the aperture is closed. The recording medium, whether it’s film or digital, must fit within the usable portion of the image circle:

fov1 (1)

Here the largest rectangle that fits within the usable image circle is the film/sensor size – making a large image circle is expensive, and so the lens is designed to create a usable circle only as big as necessary. Note that in reality it’s not a crisply defined circle – more a case of a bright center that falls off to very dim the further out from the center you go.

If the same lens projects the same image circle onto a smaller sensor, all that happens is that a smaller section from the image circle is used – the lens hasn’t changed, we’ve merely cropped the image being projected by the lens and used only a portion:

fov2 (1)

From the point of view of the camera, the field of view has changed – the angle which the picture occupies within the scene.

fov3

It’s the same lens, at the same distance, but a smaller sensor means that only a portion of the original image is captured, yielding a narrower field of view.

35mm Equivalency

Photographers need an easy way to describe what’s happening here, in terms of what they see. The most accurate way would be to talk in terms of “field of view”; a 50mm lens on a 35mm sensor has a 39 degree angle of view, the same lens on a “1.6 crop” sensor has a field of view of 25 degrees. To describe this we talk about the 35mm equivalent focal length: to get a 25 degree horizontal field of view on a 35mm sensor you’d need an 80mm lens – so a 50mm lens on a 1.6 crop sensor produces an image which is considered equivalent to an 80mm lens on a full frame 35mm sensor.

 

 

 

Pick a new camera for me

Read through the Beginners Questions forum on dpreview.com and you’ll probably see one question in 4 asks “what should I buy”, or a variation on the theme. This on a site that is filled with reviews and any number of tools aimed at answering specifically this question, and yet still the posts appear day after day.

There are no bad cameras, just cameras that are somewhat more or less suitable for specific types of photography.

If you’re a true beginner and all the specifications confuse you, it doesn’t matter what you buy – they’ll all be more capable than your skills at this point. Here are some suggestions to help seal the deal:

  • Pick a price
  • Go to Amazon and use search, enter a price range that starts 15% below what you’d like to spend, and goes to 10% over. Want to spend $200? Search from $170 to $220
  • Sort by Avg User Review.
  • Anything at the top of the list with at least 30 or 40 reviews should work just fine

As an alternative, how about your local independent camera store? Not a big box that also sells appliances and TVs, but an old fashioned camera store – they still exist. Tell the helpful staff what you want to pay and handle cameras in person, then pick whatever feels comfortable to use.

Search, where art thou?

This blog started as a result of me encountering one of the most frequently asked questions on a photography forum, for what felt like the thousandth time. If you regularly visit some of the consumer oriented forums you’ll find certain questions arise, with excruciating predictability, several times each week.

This continues to puzzle me: with apparently no expenditure of effort on their part, at all, someone expects a complete stranger to give up their time answering a question that has been asked, and answered, many times before. Often many times within the last few days.

How does this thought process work? The forum features clear instructions to anyone starting a new thread to use the search to see if their question has arisen before, but they ignore that. Some complain that there are simply too many messages to read, that’s it’s too hard to find an answer. Because everyone keeps asking the same bloody questions over and over again!

In many cases merely typing the title of their post into a Google search box would yield a workable answer within the first 3 results, but even that is apparently too much to ask.

What, ultimately, will become the fate of forums if this continues? Will there inevitably be an evolutionary process where those seeking knowledge gain it, share it, become frustrated and leave – at an ever increasing pace?

Filters, the great debate

Right up there in the list of most re-asked questions seems to be “what UV filter should I buy”.

There are only really two camps to this debate: every lens needs a filter and no lens needs a filter. I’m firmly in the later camp: UV filters are a waste of money and will quite probably degrade your image quality. There are specific lenses (the Canon 100-400 for example) which exhibit quite predictable image quality degradation when using certain filters for protection.

But what about scratches to the front element of my lens?
How often do you worry about scratches to the windshield/windscreen of your car? Think about the constant abuse your windscreen takes, can you still see through it? Do you know how hard glass is, and what it takes to scratch it? Glass is very, very hard and extremely difficult to scratch. Even if you could easily scratch the front element it wouldn’t matter:

http://www.lensrentals.com/blog/2008/10/front-element-scratches

What if I drop my lens, or bump into something?
Then your extremely thin filter will break, and the small, hard, sharp pieces will be ground into the front of your lens.

I’d rather clean the filter than my lens
Why are you needing to clean either with any regularity? Every now and then, probably no more than once a year, it occurs to me to look at the front element of my lenses. I’ll probably dust them off with a lens brush. End of story.

What about protection?
Use a rigid lens hood at all times. Unlike a filter it will improve image quality and physically protect the front of your lens from bumping into things. Digital sensors are not sensitive to UV, so there’s no need to filter it out.

What about special effects?
With only a couple of exceptions you can recreate just about any effect imaginable during post processing, with the added benefit that you can experiment with different effects on the same scene without retaking the photograph.

The exceptions? You cannot reproduce the effect that a polarizing filter has on reflections in post processing, neither can you block highlights to the same degree that a graduated neutral density filter will.

So: save your money, use a lens hood, and forget about useless filters.