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Esteemed Contributor
Posts: 3,831
Registered: ‎06-11-2013

Re: Which lens to get? 24-70 f/4 vs 24-105

@Ltc808 wrote:
Thanks! I was looking into the rokinkon 14 mm but was a little worried since it's completely manual. I read some things about how to use manual lenses so hopefully I'll get the hang of it.

Do you know what the 16-35 f/2.8 ii is good for? I figured that 2.8 would be low enough for astro shots. And is it good for landscapes?


I have the Canon EF 14mm f/2.8L II USM... but even though this is an auto-focus lens, when I use it to image the sky, it has to be focused manually.  Stars just don't offer enough contrast for the auto-focus system to lock on and focus.  


In other words, lack of auto-focus wont be a problem and you'll be usin f/2.8 anyway to get more light.


To focus.


1)  Point the camera to an area where you can see some bright stars (even if that's not the part of the sky you want to image.) This is because when "anything" in space is focused... then "everything" in space is focused.  There is no difference between focusing on, say, the moon... vs. focusing on stars.   You just need something that will make it easier to achieve accurate focus and it's easier when you have some bright stars.


2) Switch on "live view" mode (you wont use the viewfinder to focus).


3) Canon cameras have a feature called "exposure simulation" in liveview and this feature is enabled by default (you can turn it off in camera settings, but it's probably on if you haven't disabled it).  This means that as you change the exposure settings, you'll notice the liveview preview on the LCD screen getting brighter and dimmer.     Crank your exposure to the max by maxing out the ISO and setting the exposure duration to 30 seconds.  Even though this is NOT the exposure you plan to use when you shoot your image, it will make it easier to focus.  Tip:  I save this to a "Custom" setting on my mode dial so I can flip between a max exposure and the imaging exposure I plan to run just by turning the mode dial.


4) Use the 8-way navigator to move the box on the live-view screen to an area with some bright stars and then zoom in to 10x magnification.


5) Make sure the lens is in manual focus mode (if it's an auto-focusing lens) and then carefully adjust focus while watching the stars.  Try to adjust focus to make the stars as tiny and as pin-point as possible.


6) Take a test exposure for a few seconds (at this max exposure setting.)  Inspect the image to determine if you are satisified that you've nailed the focus.  


You may iterate between 5 & 6 a few times, but time spent here is a good investment of time.  It's frustrating to do a long run of image captures, get back home, start to process your images on a large computer screen, and then realize that you missed focus and all your images are a bit soft.


7) Once you are satisfied with your focus, return the camera exposure settings to whatever you plan to use to capture your shot and re-point the camera to whatever composition of the sky you intend to capture (just be careful not to touch the focus ring on the lens.)


Canon cameras also have a feature called "long exposure noise reduction" which causes the camera to take a "dark" frame (image captured while leaving the shutter closed) immediately following a "light" frame (a normal shot).  The "dark" (which contains noise) is then subtracted from the "light" frame ... resulting in an image that has less noise.  This sometimes scares camera owners who may be taking very long exposures for the first time ... into thinking the camera is broken.  They see the exposure time is complete and yet the camera wont let them do anything (that's because it really is still busy.)



As for your question on f/2.8 and the 16-35mm... yes f/2.8 is certainly good enough for astro-shots.  I'd say that's a fairly common focal length ... particularly if the camera is not on a "tracking" head (which would allow siginificantly longer exposure times.)


The 16-35mm is a great focal length range for landscapes.  I've seen some gorgeous landscape photos taken at 16mm.


You can use a site called to view images shot with any lens & camera combination.   They don't host photos... they search and index images found on Flickr which include all the image EXIF data indicating what camera, lens, and exposure settings were used for the shot.   


This means you can tell them you want to see images shot with a Canon EF 16-35mm f/2.8 while mounted on a full-frame camera body (you could even limit the search to a specific focal length range in the zoom range... or limit it to only specific f-stops, etc.


Here's a link to a sample of images shot using the 16-35 f/2.8 on full-frame bodies.  I didn't limit the focal length range or f-stop.


You can use this to quickly get an idea of how certain lenses are used... what sort of results they get... etc.  (just be warned that since these are posted to Flickr, they are probably not "straight out of the camera" images.  Almost certainly some post processing has been applied.)


Tim Campbell
5D III, 5D IV, 60Da
Occasional Contributor
Posts: 14
Registered: ‎10-22-2015

Re: Which lens to get? 24-70 f/4 vs 24-105

Thank you! I'll definitely be using your advice/instructions for Astrophotography. I also checked out that link you posted and it's great. It's nice being able to check out different photos for different settings
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