Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.

mollyc

macrumors G3
Original poster
Aug 18, 2016
8,014
49,938
On the POTD thread, we were having a discussion about squawk’s photo of a statue in the city. Another poster made a remark that it had great compression. We then started a discussion that seemed like it was a bit larger than the POTD thread, so I thought I would move it to a new thread.

What is lens compression? It’s the effect that a telephoto lens has on the background of an image, flattening it out (compressing it) towards the viewer. It tends to make the background objects of an image larger (which has always personally confused me why it’s called compression because to me, compressing something makes it smaller). Using a telephoto lens is a great way to minimize distractions in your images because it essentially removes all the things in the background.

In squawk’s image, he used a 44mm focal length on a crop body. This has the effect of making the lens act as a 66mm due to the crop factor. However, note the focal length itself has not actually changed. It is still a 44mm lens, but the field of view has changed to the equivalent of 66mm.

The focal length of a lens, expressed in millimeters, is the distance along the lens's optically central axis (beginning at the rear nodal point) to the image plane in the camera (often illustrated by a "" on the top plate of a camera body) when the lens is focused at infinity. The image plane in the camera is where you will find your digital sensor or film plate.

The focal length does not change, regardless of the sensor size behind the lens mounted in the camera. The “crop factor” comes from using a smaller sensor inside the circle area of the lens shape. The field of view has changed, but not the optics of your lens.

Here shot Gnomester with my 70-200 lens. (All images shot on a Nikon Z6ii at ISO 320, f/4, 1/320. They are not edited in any way, other than my import preset to Lightroom and any mentioned cropping. All images were shot with either a Nikon 70-200 F mount or a Nikon 24-70S Z mount.) These two images are zoomed out to 200mm (exif actually reads 180mm), but the bottom image is in FX (full frame) mode, and the top image was shot in DX (crop) mode. Note, the images are exactly the same, other than the outside of the image being cropped out from the DX image.


I8hFDPllfYKXj4IugL-vunU0AJIrvk7kBajWZE781falIhwGSVoNqbl3wOX61e38zEpu5SvGEWcniUQK4RYdNEi3cKFFxMfmn7qL22PXT5D2sIOzNfbHNV0IeWMUzN_ohts2eZoG



So while the DX image has an equivalent crop factor of 270mm (it feels “zoomed in” to 270mm), it is actually still a 180mm image; but the sides have been cropped out.

Now, for the purposes of squawk’s image, I asserted that there was little to no compression in the image because the background was small and faded away from the subject. This is because it was shot in the “normal” range. Typically wide angle lenses are considered to be 35mm or wider, a normal lens is in the range of 50mm, or what our eyes see naturally. Telephotos are generally considered to be 85mm and up.

It’s true that if you stay in one position and change your focal length, you can merely crop your wide(r) angle image in post production and make it seemed zoomed in. Here I have done just that. The field of view is the same, and Gnomester takes up the same amount of space in both images. The bokeh quality is a bit different, but non photographers wouldn’t really notice that too much.


gJwzgZ1O0tHjoD32hc_p-oxvOxxpYKC8wnfiGPQOQy974otXnHoGKGF5_YU_FyyGOLDFaNJruWKMzv75lo7lAOrRVIrNIN5tCYR6YwzC0E9eKtX3hxb7wWpGajwtLVklG8HTyTw5



You can even crop from 24mm to 200mm but you start to get image degradation as you are cropping out too many pixels. Still, the field of view is the same. You can see the sides of the garden beds in each image, and the same amount of seat for Gnomester.


JSHNgfNbggqEkGY0tQyKp1oz0t0yXSoLgA83MAtXvdCnA21_FemMH91579HeuDOeJ9p1FLImKFYjKBqeopLhchscaMXWrtTcH8tJteQQyDzJFe2DBkUH5mI5CSa1rmoVP5g8dp0B




But…look at the backgrounds of those images cropped in post. They have quite a bit of detail in them, the bokeh isn’t as smooth (even at the exact same aperture). This is because telephoto lenses compress the background. Let’s look at the background of the full 24mm image, uncropped. Sure, as indicated above, I can phyiscally crop the images in post to get to an equivalent 200mm, but I am losing oodles of my image.

This is the full image of the 24mm zoomed in above.

pvBNW5w3zKLnNT9xwX26XmNEYb3Ofrm7O4u5CzT65K34DN0yyooeIQy-0g--2iigLIzb5LsjxOQFnGrYKhytxHf3glMsQy_b85uFT3tqjBlgxl7z9VSfcueziwV7uTnPN4e6nwgl




But what does the image look like at 24mm, if I want to approximate the field of view in camera? Because moving your feet closer to your subject makes a lot more sense than cropping a photo by more than half its pixels.

TG_Njm7EzMpG0v6Tr98bNAhhA69cRUyZc3Twh1a-PPf5WmJ7yu5pCEnEmUiE-1WI0cWd4h6mat0nyOIxof22U0zkHiWmoyZ5w8g03hh8HgAiYAsgwXMtP4rfiebxaXX9bLYLV71C


Here we now have Gnomester taking up the same amount of space in both the 24mm image and the 200mm image, but the backgrounds look very different. And this is what we mean by background compression from a telephoto lens. In the 24mm image you can see my house, my neighbor’s shed, part of my grill, and the house far in the background about 100 yards away. In the 200mm image, all that background area has been compressed, flattened, and moved forward so that all you see is the grass area, with no other distractions.

The bokeh becomes larger and more luminous, the shadow lines softer, blurrier.


yVCSPlXjydavCpRpKARUmzk93-Ep2V6WOihdbHSdvyEWg9Lg5s-oo6_P5DwM495dj73oiZi3acN9wg30GSI2ilDQ2tkCKWYurtVt4jgs7Ykib52ew_8gGbOjwwQy0cEf23pNwnn5



I look forward to any other samples and a lively discussion. ?
 
Last edited:

USAntigoon

macrumors regular
Feb 13, 2008
246
973
Rochester Hills, MI
I am not going to argue with the technicalities…In fact one needs first to decide what your picture composition will be, how are you going to frame it..
Once that is somewhat defined, a choice of lens, wide angle or tele, prime or zoom, will become the next step..
Yes, one has to understand DOF, aperture and the background effect (aka bokeh) these different lenses will create.. The word background compression is a misleading choice of words (just my opinion)
Further, it doesn’t make sense to use a wide angle lens and do heavy cropping afterwards..
 
  • Like
Reactions: Grey Beard

mollyc

macrumors G3
Original poster
Aug 18, 2016
8,014
49,938
I am not going to argue with the technicalities…In fact one needs first to decide what your picture composition will be, how are you going to frame it..
Once that is somewhat defined, a choice of lens, wide angle or tele, prime or zoom, will become the next step..
Yes, one has to understand DOF, aperture and the background effect (aka bokeh) these different lenses will create.. The word background compression is a misleading choice of words (just my opinion)
Further, it doesn’t make sense to use a wide angle lens and do heavy cropping afterwards..

Well, I didn't invent the terms, I'm just using them as they are typically thrown around on photography websites. ? And on the POTD thread, the word compression was used specifically to describe an image.

The takeaway I wanted most people to have is that you can have the subject of your take up the same amount of the frame, but the background can look vastly different based on what focal length was used.

This principle is the same one that causes most people to choose a longer length lens for portraits, as wide angle lenses distort faces when used for headshots. The nose become disproportionately large on the face using a wide lens, when the head takes up the same amount of space in an image if say, an 85mm lens is used.

The other takeaway is that a crop body doesn't change the actual focal length used, it just crops out the edges so that the subject seems to take up more space than it really does. It doesn't make your focal length longer.
 

Apple fanboy

macrumors Ivy Bridge
Feb 21, 2012
56,612
55,277
Behind the Lens, UK
Well, I didn't invent the terms, I'm just using them as they are typically thrown around on photography websites. ? And on the POTD thread, the word compression was used specifically to describe an image.

The takeaway I wanted most people to have is that you can have the subject of your take up the same amount of the frame, but the background can look vastly different based on what focal length was used.

This principle is the same one that causes most people to choose a longer length lens for portraits, as wide angle lenses distort faces when used for headshots. The nose become disproportionately large on the face using a wide lens, when the head takes up the same amount of space in an image if say, an 85mm lens is used.

The other takeaway is that a crop body doesn't change the actual focal length used, it just crops out the edges so that the subject seems to take up more space than it really does. It doesn't make your focal length longer.
Very interesting discussion. Thank you for taking the time to put it together.
 

ericgtr12

macrumors 68000
Mar 19, 2015
1,774
12,175
Great write up @mollyc, I won't get into the back and forth on the use of the word compression, just to say it's a well known term when it comes to zooming in in the world of photography.

I personally go out of my way to take shots with a lot of compression in them, it's one of the few things these days that can separate your photos from the millions of others out there. It's also a lot of fun to find subjects that work well with it.

The moon is an example taken 9 miles away from the Golden Gate bridge as the supermoon was setting back in 2021, this was a planned shoot way in advance but I used a 600mm lens to capture the breadth of it. Yes, it is compressed and it also gives a true to life perspective. Trying to take this with a cell phone will show nothing but a white dot.

The Bay Bridge shot is taken at street level while at full zoom at 200mm with the street in in the foreground giving it the actual effect/perspective.

IMO this is just creative use of your gear and a part of the art of photography that we should both learn and practice. Additionally, with today's higher resolution cameras the ability to crop for this effect using a wider lens is another tool we can use to pick and choose what part of the shot we want to use in post.
 

Attachments

  • IMG_0911.jpeg
    IMG_0911.jpeg
    139.4 KB · Views: 101
  • IMG_2106.jpeg
    IMG_2106.jpeg
    301.4 KB · Views: 137
Last edited:
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.