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Mặc dù ý tưởng này là hữu ích, nhiều máy chủ thực hiện chuyển tiếp mà không cần chứng thực người gửi, cho kẻ gửi thư rác mở mà họ cần. Chúng tôi nói về thư rác và những gì bạn có thể làm gì về nó sau này trong chương này. Điều tồi tệ nhất về SMTP nguyên là nó chấp nhận các tin nhắn văn bản, không phải là tập tin nhị phân. | Chapter 17 Digital Cameras Video Capture and DVDs 281 There s another conclusion you can draw from Table 17-1. Knowing that computer monitors typically deliver 90 dots per inch or more corresponding to 0.28 mm pitch or finer it s apparent that even the least expensive digital camera should have enough resolution for pictures destined for Web pages. Ultimately digital cameras are still cameras so the issues that are important for film cameras are equally important for digital ones. You still care about the focal length resolution contrast and speed of the lens. You still have to have enough light to form a picture so the effective film speed matters. You still have to use a flash in low-light situations so the synchronization and control of the flash unit and the time it takes to recharge are important. You have to worry about parallax between the lens and the viewfinder and may want features like macro focus and a self-timer. These are cameras only the film is different. Choosing a digital camera There s more to choosing a digital camera than getting the greatest number of pixels you can afford. You can get over 5 megapixels in a reasonably priced camera enough to do an 8x10 print with good quality if you fill the frame. Getting a photo you d want to print that size however requires you look at some other factors Auxiliary lenses Some cameras offer detachable lenses or add-on lenses you can slide over the standard lens. You can add telephoto or wide-angle capabilities to your camera with auxiliary lenses letting you fill the frame for shots you d otherwise miss. Check how the lenses fit on the camera because some of them block the viewfinder and leave you no choice but to frame the picture with the LCD panel. Battery system The power drain from a camera is relatively fixed so the battery system in your camera determines how long you can shoot before you have to recharge or change batteries. Cameras that let you replace batteries with standard types let you keep .
