If you want the SLR camera with a zoom/larger lens quickly available for a shot the KISS solution I have been using for years is to attach the camera straps high on the backpack and let it hang with the top of the camera towards the chest. You then put your sternum strap over the camera straps to stop the camera from dangling around. To take a photo you just open the sternum strap and rise the camera with crossed camera straps. Crossing the camera straps can make them a bit tight when shooting, but when you get the length right its no problem and you can still make them a bit tight to reduce camera movement.
The key is to attach the camera straps on the pack level to the shoulders or even better a little bit above. Not to the shoulder straps or have it just hanging around the neck. This will transfer the camera load to the hip and at the same time make a much better job than load lifters to pull the pack towards you, so it will actually reduce the load on your shoulders. In my opinion, more comfortable than carrying the camera in the pack. Of cause, you need a pack with reasonable load transfer to the hips that reaches or preferable goes above your shoulders. ( I usually run the camera strap through a loop on the shoulder straps to make sure it stays on the padding without fixing it to the straps.) Hanging the camera with the top towards the chest will be much more comfortable for the chest.
I am now combining this with a Zpacks poncho and and a good rain hat (a Sou’western at the moment , the best foul whether rain hat ever designed, Sealskinz have one that is not to heavy). The idea is that having a good rain hat and the main ventilation on sides will make it possible to avoid the normal leak down the chest so you can have your camera there to keep the camera warmer than the air, reasonable dry and available for a shot for most rains. If its really windy and rainy, it will not work, but then you would probably need an water protected camera just to make the shoots as well. The camera on your chest should increase ventilation under the poncho as well. Most times the poncho should provide enough warmth to avoid instant lens condensation I hope, time will tell.
Its early days for the poncho but it seems to work reasonable well so far. It would had been better with a slightly longer front zipper when you take out the camera for a photo, but I don’t find it a major hassle. Since I use poles, I have also made “rail arms” that I attached to the poncho, this also seems to work really well.
(I am not a great believer in WPB, being in Scandinavia I need to be able to handle 5 days or more of rain. The main reason for the rain hat is that using a hood when raining would very much restrict my experience during rainy days by making me only look down/forwards – that is not at all necessary just because it rains.)
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