Comments on: What Is Focal Length? (And Why It Matters in Photography)

By: Steve

Steve — Mon, 23 Aug 2021 20:45:06 +0000

Sorry if this sounds critical but, as an optical engineer, I can tell you your definition of focal length is misleading. The who discussion of the point of convergence is simply wrong. That point of convergence is, in a well designed lens, where the aperture is located. If the distance from it to the sensor happens to equal the focal length of the lens, its a fluke.

All you need to say, and should say, is ‘the focal length of a lens is identical to the focal length of a single lens that would provide the same field of view’

By: Amit

Amit — Sun, 12 Jan 2020 07:18:17 +0000

I have a clarification on Section 3, are you moving the camera back to compensate for the focal length increasing with each lense?

Thanks!

By: Sebastian

Sebastian — Sat, 30 Nov 2019 11:23:03 +0000

This was such an amazing explanation. Thank you so much. I had been struggling with what this means

By: Liz

Liz — Sun, 27 Oct 2019 12:44:08 +0000

Thanks a lot for this article it is explicit, simple and to the point easy to understand. I was having problems with the cropping and the focus thanks this was very helpful keep posting articles like this I will be your follower. thanks

By: Zombi Bikini

Zombi Bikini — Mon, 23 Oct 2017 16:23:00 +0000

How is that possible for a lens to completely change the appearance like that? My mind is aboggled.

By: Greg Conquest

Greg Conquest — Sun, 10 Sep 2017 23:51:00 +0000

Shooting on a crop sensor has what’s known as the ‘crop factor’. This
essentially means that any full frame lenses (EF, FX, etc.) that you put onto a crop sensor body will have a cropping effect.
The crop factor is approximately 1.6. In real terms, this means that if you shoot at 35mm, the actual result will be closer to a 50mm image.

This is in correct, at least in part. I put a 35mm Takumar lens (FX, m42-mount) onto a K&F m42-m4/3 glass-less adapter (no lenses inside, just a metal tube) and this onto my Olympus E-M5 MarkII micro-four-thirds camera, and the field of view was not the same as when using a native 17mm* lens on that camera (no adapter needed). The field of view with the 35mm FX lens was the same as a native, m4/3 35mm lens (I used my kit lens zoomed to 35mm).
APS cameras have a 1.5X crop factor; m4/3 cameras have a 2X crop factor.

By: Josh

Josh — Sun, 28 Dec 2014 13:19:37 +0000

In reply to Stephan. I'd personally go for the 35mm!

By: Stephan

Stephan — Fri, 26 Dec 2014 00:21:46 +0000

Hello and Merry Christmas
Thanks for the article; looking around and noticing the quality of your posts it becomes clear that I will spend a lot of time on your site.
I do have a questions though. I’m a beginner and putting together an equipment list. As far as the camera is concerned I decided on Nikon D5300. It is still unclear to me what to pick as prime lens for this camera body. I was thinking to get the AF-S NIKKOR 50mm f/1.8G. My confusion is in regards to the focal length. Since this camera has a 1.5 crop sensor, should I get the AF-S DX NIKKOR 35mm f/1.8G instead?
Thank you

By: Tamsin Maund

Tamsin Maund — Tue, 29 Jul 2014 13:32:23 +0000

Message to JMM of May 24, 2011 at 9:09 pm – You have grammatical errors in your post. Ironic.

By: Josh

Josh — Mon, 21 Nov 2011 13:49:52 +0000

In reply to gepaza. No, I see your point and agree with it, I'll try and find something to replace it with.