Doing Alaska on a cruise means that you need to be cognizant of the weight of your gear. Not because the ship will sink under the weight of a few extra lenses but, because whenever you leave the ship - all the gear you will have is the gear you are able and willing to carry. You won't be able to go back and forth to the ship to grab that extra lens which you didn't want to carry when you left the ship.
This is a lot different from flying to Alaska, renting a vehicle and staying in lodges, hotels or motels. You can keep your gear in the trunk of your vehicle (I tend to rent vehicles with trunks because of the extra security factor) and are usually closer to the gear if you need it. Still, with airline weight restrictions, you can't go overboard with gear either.
The lenses which you will need really depend on where you are going and what you are going to do. If wildlife is a major part of your interests, you should have a lens of at least 400m on the long side. if you are going to take the trip to Denali, you could use a lens in excess of 400mm since much of the wildlife viewing in that area is done from long distances. However, few persons except professionals or truly devoted amateurs are willing to carry the long lenses and heavy tripods needed for Denali wildlife photography and these photographers don't go there on a tour.
If I were going to take a cruise to Alaska, I would prefer two cameras with two lenses: 17-55mm f/2.8 IS and the 100-400mm f/4.5-5.6L IS. I would team these up with a good monopod and with a flash. These two lenses would suit me very well especially since I am not a great fan of ultra wide lenses for landscapes. However, if you like the wide stuff, throw in a 10-20mm Canon, 12-24mm Tokina or the Sigma equivalent. If you don't have a mid-range zoom with a wide side; then a three lens setup with the wide angle, a mid-range zoom in the area of 24-28mm to 70-75mm and the 100-400mm would do very well.
Of course, if you are not willing to lug two cameras, a single camera with the 17-55 and 100-400 lenses would even be a lighter outfit. However, this would mean changing lenses in the field and not having a spare camera if the one camera bites the dust. Remember: Murphy's Law of Photography states, "The probability of a camera's failure is in a direct relationship to the importance of your shooting and in an inverse relationship to the availability of backup equipment."
You can "rent" a lens like the 100-400mm f/4.5-5.6L IS from eBay! Simply buy a used one in good condition, pay for it with PayPal credit and then sell it on Ebay when you return from your trip. You will not have to make a payment until you have sold the lens and paid off the credit line.
You could of course, carry your 70-200mmm f/2.8L and a 1.4x TC which would bring you to 280mm. I would not, however, shoot on a once-in-a-lifetime trip using a 2x TC because I don't like the imagery a 2x provides.
I am in a flux as to what equipment I want to bring. I have settled on my 30D and 40D cameras with 17-55mm f/2.8 IS, 70-200mm f/4L IS and 300mm f/4L IS lenses and 1.4x TC.
My dilemma is whether I should bring my D60 which has been converted to full time infrared. The problem with this camera is that it cannot use an EF-S lens such as the 17-55mm and I would need to bring a mid-range EF lens like the 17-40L or the 24-70L. In retrospect, I kind of wish I had an infrared P&S instead of the D60. This would make logistics a lot easier.
The extra D60 camera and lens would be quite a bit of extra weight. However, since I will just need to carry the gear through airline terminals and to and from my lodge or hotel rooms, I will probably bring the infrared camera and extra lens. I know that if I don't, I will see all sorts of uses for the camera that has been left at home. I will probably also throw in my 12-24mm Tokina for the same reason.
This is my tentative gear. The trip will be a combined fishing and photography adventure and the Nikon Coolpix is to throw in my fishing vest (with a waterproof plastic housing). I certainly don't want to take a dunk while wade fishing in an Alakan stream wearing a 30D or 40D plus lens or lenses.
Remember, i am not going to carry this on my back as I am shooting - the pack is just to transport my gear to Alaska and to store it when I am there. I would rather carry the gear there and not use it than not have it and need it.
I will also take a monopod and a tripod.
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